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LIBRARY CATALOfUTK SLIPS. 

United States. Deparlimnt of the hilcriur. ( r. ,S. yeoloi/U'dl surveii.) 

Department of the interior | — | Monograi)lis | of tlie | Unit(^(l 
States geological survey | Volume XXIII | [ Seal of tlie depart- 
meut] I WasUiugtoii | government printing ofiHce | 1S91 

Second tUle: United States geologieal survey | .1. W. Powell 
director | — | Geol gy | of the | Green monntain.s | in | Massa- 
chusetts I by Raphael Pumpelly, J. E. Woltf, and T. Nelson Dale | 
[Vignette] j 

Washington | government printing office | 1894 

4°. XIV, 206 pp. 2;! pi. 



Pumpelly (Raphael) and others. 

United States geoh)gical survey | .1. \V. Powell director | — | 
s Geology | of the | Green mountains | in | Massachusetts | liy 

Z Raphael Pumpelly, J. E. Wolff, and T. Nelson Dale | [Vignette] | 

a Washington | governnu'ut printing office | 1894 

5 4°. XIV, 2U6 pp. -23 pi. 

[United States. Deiiart ineni of the intcrinr. (P. !i. rjeoloijical .iiiroey.) 
Monograph SXllI.j 



United States geological survey | .1. W. Powell director | — | 
Geology I of the | Groen mountains | in | Massachusetts | by 
Raphael Pumpelly, J. E. Wolff, and T. Nelson Dale | [Vignette] | 

Washington | government printing office | 1893 

4°. XIV, 20li pp. 23 pi. 

[UxiTEi) States. Drpartinent u/ the interior. {U. *. ijeoloijical aiuoey.) 
MouograpU XXIII. ] 



^DVERTISE]S4:ElSrT, 

[Monograph XXIII. 1 



The publipations of the United States Geoloj^if nl Survey are iasned in apcordance with the statute 
approved March o, lS7iJ, which dechires that — 

" The public-atiiiiis of the Geological Survey shall consist of the annual report of operaMous, geo- 
logical and economic maps illustrating the resources and classification of the lauds, and reports upon 
general and economic geology and paleontology. The annual report of operations of the Geological 
Survey shall accompany the annual report of the Secretary of the Interior. All special memoirs and 
reports of sai<l Survey shall be issued in uniform cjuarto series if deemed necessary by the Director, but 
otherwise in ordinary octavos. Three thousand copies of each shall be published for scientific exchanges 
and for sale at the ])rice of pulilication ; and all literary and cartographic materials received in exchange 
shall be the property of the United States and form a part of the library of the organization : And the 
money resulting from the sale of such publications shall be covered into the Treasury of the United 
States." 

The following joint resolution, referring to all goverument publications, was passed by Congress 
Jnly7, 1882: > i j s 

" That whenever any document or report shall he ordered printed by Congress, there shall be 
printed, in addition to the number in each case stated, the ' usual number ' (1,900) of copies for binding 
and distributiou among those entitled to receive them." 

Except in those cases in which an extra number of any publication has been supplied to the Sur- 
vey by special resolution of Congress or has been ordered by the Secretary of the Interior, this ottice 
has no copies for gratuitous distribution. 

ANNUAL REPORTS. 

I. First Annual Report of the IJnited States Geological Survey, by Clarence King. 1880. 8 '. 79 
pp. 1 map. — A preliminary report describing plan of orgauizaticm and publications. 

II. Second Anniuil Report of the United States Geological Survey, 1880-'81, by ,1. W. Powell 

1882. 8°. . 1 V, 588 pp. 62 pi. 1 map. 

III. Third Annual Report of the Unite<l States Geological Survey, 1881-82, by .J. W. Powell. 

1883. 8°. xviii, 564 pp. 67 pi. and maps. 

IV. Fourth Annual Report of the Uiuted States Geological Survey, 1882-'83, by ,T. W. Powell. 

1884. 8'^. xxxii, 473 pp. 85 pi. and maps. 

V. Fifth Annual Report of the United States Geological Survey, 188.3-84, by .1. \V. Powell. 

1885. 8°. xxxvi, 469 pp. .58 pi. and maps. 

VI. Sixth Annual Report of the United States Geological Survey, 1884-85, by J. W. Powell. 
1885. 8°. xxix, 570 pp. 65 pi. and maps. 

VII. Seventh Annual Keiiort of the United States Geological Survey, 1885-86, by .J. W. Powell. 

1888. 8°. XX, 656 pp. 71 pi. and maps. 

VIII. Eighth Annual Report of the United States Geological Survey, 1886-87, by J. W. Powell. 

1889. 8°. 2 v. xix, 474, xii jip. 53 pi. and mpps; 1 p. 1. 475-1063 pp. 54-76 pi. and maps. 

IX. Ninth Annual Report of the United States Geological Survey, 1887-'88, by J. W. Powell. 

1889. 8^. xiii,717pp. 88 pi. and maps. 

X. Tenth Annual Report of the United States Geological Survey, 1888-'89, by .1. W. Powell. 

1890. 8°. 2v. XV, 774 p]). 98 pi. and maps; viii, 123 pp. 

XI. Eleventh Annual Rejiort of the United States Geological Survey, 1889-90, by •!• W. Powell. 

1891. 8*^. 2 V. XV, 757 pp. 66 pi. and maps; ix, 351pp. 30 pi. and maps. 

XII. Twelfth Annual Rei)ort of the United States Geological Survev, 1890-'91, liy J. \V. Powell. 
1891. 8°. 2 V. xiii,675pi). 53 pi. and maps; xviii, 576 pp. 146 pi. and maps, 

XIII. Thirteenth Annual Report of the United States Geological .Survev, 1891-';i2, by J. \V. 
Powell, 1893. 8^^. 3 V. =. . > . 



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118. .Studies in the (Structure of the Green Mountains, by T. Nelson Dale. 

The Moraines of the Mi.ssouvi Coteau and their attendant deposits, hy .James Edward Todd. 

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To THE DiRECTOI! "IF THE 

United States Geological Survey, 

Washington, D. C. 
Washington, D. C, March, 1SU4. 






U1 






^ 


Tin 






en 
















LiJ 




CE 




O 









Vol -^3 



DEPARTMENT OF THE INTERIOR 



MONOGRAPHS 



OF THE 



United States Geological Survey 



VOLUME XXIII 




WASHIIfGTON 

GOVERNMKNT FEINTING OFFICE 

189 4 



\jo ('23 



UNITED STATES GEOLOGICAL SURVEY 

J. W. POWELL, DIRECTOR 



GEOLOGY 



OF THE 



GEEEN MOUNTAINS 



IN 



MASSACHUSETTS 



BY 



RAPHAEL PUMPELLY, J. E. WOLFF, AND T. NELSON DALE. 




'sJ^ 



WASHINGTON 

GOVERNMENT PRINTING OPFICB! 
1894 



CONTENTS, 



Page- 
Letter of transmittal xi 

Preface xili 

Part I. — Geology of the Green mountains in Massachusetts, by Raphael Pumpelly. 

General description 5 

Age and structure 7 

Correlation 9 

Part II. — The geology of Hoosac mountain and adjacent territory, by J. E. Wolff. 

Introduction 41 

Topographic work 41 

Topography 41 

Description of rocks of Hoosac mountain 44 

The Stamford gneiss 45 

The Vermont formation 48 

The Hoosac schist 59 

The Stockbridge limestone 64 

Amphibolites 65 

Geology 69 

The Hoosac tunnel 69 

The region embracing the central part of Hoosic mountain 72 

The northern and eastern schist area 86 

The region south of Cheshire and of the Hoosic valley 88 

Hoosic valley schist 97 

The region around Clarksburg mountain and Stamford, Vermont 98 

General conclusions , 102 

Part III.— Mount Greylock: its areal and structural geology, by T. Nelson Dale. 

Outline of this paper 125 

Historic 131 

Physiographic 133 

Structural 136 

Types of structure 138 

Correlation of cleavage and stratification 155 

Pitch 157 

Structural principles 157 

Structural transverse sections 158 

Lougitutional sections 175 

Resumd, structural 177 

Lithologio stratigraphy ,,..,, 179 

V 



VI CONTENTS. 

Page. 

Pctrograpliy 181 

Areal and structural 191 

Relation of geology to topography 192 

Appendix A: Stone hill near Williamstown 197 

Appendix B: New Ashford 202 



ILLUSTRATIONS. 



Page. 
Pl-ATK I. Map of Greylock and Hoosao mountains Frontispiece. 

II. General map showing the relation of the Greylock series to the Hoosac mountain 

rooks 10 

III. Structural relations of the Hoosac series 14 

IV. Detailed map of western crest and slope, Hoosac mountain 40 

V. Geologic profiles, Hoosac mountain 70 

VI. Geologic profiles, generalized, Hoosac mountain 80 

VII. Thin sections, white gneiss 110 

VIII. Thin sections, white gneiss and albite schist 112 

IX. Thin sections, diorite and amphibolite 114 

X. Thin sections, quartzite conglomerate and crumpled metamorphic conglomerate 116 

a . View north over crest of Hoosac mountain 118 

■ b \ Profile of Hoosac mountain from Spruce hill south, looking west . . ^ 118 

XII. Mount Greylock, eastern side 130 

XIII. Mount Greylock, western side 132 

XIV. Southern summit of Mount Greylock 134 

XV. Southern side of Mount Greylock 136 

XVI. Southern end of Ragged mountain 160 

XVII. The north-south part of Hopper 192 

XVIII. Greylock sections, A, B, C, D 

XIX. Greylock sections, E, F 

XX. Greylock sections, G, H, I 

XXI. Greylock sections, J, K, L, M 

XXII. Greylock sections, IJ, O 

XXIII. Greylock longitudinal sections, P, Q, R 

Fig. 1. The Stamford dike, showing the Cambrian conglomerate deposited in dike fissure 11 

2. The Stamford dike, plan 11 

3. Correlated columns of the Hoosac and Greylock rocks 13 

4. Anticlinal arch across Hoosic river between North Adams and Briggsville 15 

5. Ideal section east of Cheshire, showing lateral transition of limestone to schist 17 

6. Diagram of structure, summit of the Buttress 22 

7. Crumpled structure in albite-schist 23 

8. Map showing the varying character of Cambrian rocks around the Hoosac core 31 

9. View from Hoosac mountain 42 

10. Profile of Hoosao mountain ( western crest) 43 

11. Profile of Hoosac mountain ( western slope) 44 

12. Granitoid gneiss 45 

VII 



a 



a, 
o 



VIII ILLUSTRATIONS. 

Page. 

Fig. 13. Metamorphic conglomerate showing crushing 48 

14. Metamorphic conglomerate, showing shape of pebbles 49 

15. Metamorphic conglomerate; flattened pebbles 50 

16. Metamorphic conglomerate ; round and flat pebbles 51 

17. Metamorphic conglomerate, banded variety 53 

18. Metamorphic conglomerate, typical 55 

19. Metamorphic conglomerate, showing large pebbles 57 

20. Conglomerate ; cliff 58 

21. Albite-schist, Hoosac schist 59 

22. Albite-schist, Hoosac schist 61 

23. Albite-schist, Hoosac schist '. 62 

24. Mount Holly amphiliolite _ g5 

25. Mount Holly amphibolite .-...- _ 66 

26. Mount Holly crumpled amphibolite 67 

27. Contact of grauif old gneiss and metamorphic couglomerate 73 

28. Contact of grauitoid gneiss and quartzite, Stamford dike, looking north 100 

29. Contact of grauitoid gneiss and quartzite, Stamford dike, looking east 101 

30. Northwestern side, Mount Greylock 136 

31. Albitic sericite-schist in contact with limestone 138 

32. Sericite schist with two foliations, in contact with limestone I39 

33. Sericite schist ; specimen with two foliations 139 

34. Thin section illustrating origin of cleavage I.j0 

35. Sketch of ledge south of Sugarloaf, showing cleavage in both limestone and schist 140 

36. Limestone block with cleavage ; Sugarloaf 141 

37. Limestone ledge with cleavage ; east of Sugarloaf 141 

38. Weathered limestone from East mountain 142 

39. Polished surface of limestone shown in Fig. 38 142 

40. Weathered limestone with mica in cleavage planes 143 

41. Specimen of sericite-schist showing stratitication and cleavage. Bald mountain 144 

42. Specimen of sericite-schist showing only cleavage, Symonds peak 144 

43. Section of specimen shown in Fig. 42 I45 

44. Section of specimen of sericite-schist, top of Mount Greylock I45 

45. Microscopic drawing of sericite-schist, top of East mountain 146 

46. Specimen of sericite-schist, one-fourth mile south of Mount Greylock 147 

47. Diagrams showiug relation of quartz lamiuiv to cleavage 148 

48. Ledge of sericite-schi.st, junction of Gulf and Ashford brooks 148 

49. Part of ledge shown in Fig. 48 I49 

50. Section of sericite-schist with quartz lamin;e ; from Goodell hollow 150 

51. Ledge of mica-schist in Readsboro, Vermont, svith ([uartz in both foliations 151 

52. Sericite-schist with two cleavages, Goodell hollow 1,52 

53. Section of sericite-schist, one- fourth mile south of Greylock summit 153 

54. Sericite-schist, one- fourth mile southwest of Greylock summit 154 

55. Diagram showing fault between schist and limestone t 154 

56. Section of sericite-schist. Bald mountain spur 155 

57. Diagram showing relation of cleavage to stratification 156 

58. Diagram showing relation of cleavage to stratification 156 

59. Quartz laminae in schist, west side of Deer hill 157 



. ILLUSTRATIONS. IX 

Page. 

Fig. 60. Minor pitching limestone folds 157 

61. Cross-section G 160 

62. Section of syucline at soutli end of Ragged mountain 161 

63. Cross-section H 166 

64. Cross-section I 166 

65. Cross-sections A, B 169 

66. Cross-section F 171 

67. Cross-sections J, K, L 172 

68. Structure in schist, south side of Saddle Ball 173 

69. Cross-sections M, N, O 173 

70. Structure in schist, west of Cheshire reservoir 174 

71. Longitudinal sections P, Q, R 175 

72. Continuity of the folds ou the Grey lock sections 178 

73. Albitic sericite-schist : typical Greylock schist 188 

74. Outline sketch of Round rocks 194 

75. Sketch of Greylock mass from southwest 195 

76. Cross-sections S, T, U. Stone hill 198 

77. Sketch of protruding limestone anticline. New Ashford 202 

78. Diagram map of Quarry hill. New Ashford 202 

79. Cross-section of Quarry hill. New Ashford 203 



LETTER OF TRANSMITTAL. 



Department of the Interior, 

U. S. Geological Survey, Archkan Division, 

Newport, E. L, January 18, 1892. 

Sir : I have the honor to transmit herewith a memoir on the Greology 

of the Green mountains in Massachusetts. 

Your obedient servant, 

Raphael Pumpelly, 

Geologist in charge. 
Hon. J. W. Powell, 

Director U. S. Geological Survey. 

XI 



PREFACE. 



The following memoir is the result of the fieldwork of the Archean 
Division of the U. S. Geological Survey in northwestern Massachusetts, 
during the years 1885, 1886, and 1887. 

The conclusions put forth were all arrived at before 1888, but the 
publication of them was delayed until they should be either confirmed or 
corrected by the results of further study in southwestern Massachusetts 
and in central Vermont. 

The progress of our survey of western New England has fully con- 
firmed our interpretation of the facts observed in the Hoosac mountain and 
Grreylock area. It has been our intention to keep wholly clear of the 
Taconic controversy, and to confine our efforts to accurate study and inter- 
pretation of structure. In the first part I have given a statement of the 
sequence and bearing of the results and have advanced some theoretical 
views in explanation of the sudden disappearance of the Lower Silurian 
limestone against the western base of the Grreen mountain anticline. I 
have also advanced a hypothesis, supported by observation in the northern 
and southern Appalachians, to explain (through the presence of a previously 
deeply disintegrated land surface) the apparent conformable transition 
between Archean or pre-Gambrian gneisses and Cambrian quartzite. This 
almost insuperable difficulty is met with in many of the great crystalline 
areas of the world, in passing from Archean or eruptive masses to the clastic 
crystalline schists. 

The second part treats of Hoosac mountain — the central or crystalline 
range of the Grreen mountains. The field work was performed by Dr. J. 
E. Wolff, Mr. B. T. Putnam, and myself. The analysis of the results, the 
petrographic study, and the presentation are by Dr. Wolff. Mr. Putnam 



XIV PREFACE. 

had contributed largely to the sum of the work. His early death in 1886 
deprived the Survey of one of its most accurate and thouglitful geologists. 

The third part deals with the Grreylock synclinorium — made up of the 
Cambrian-Silurian quartzite, limestones, and schists, which are the offshore 
time equivalents of the white gneisses and schists of Hoosac xnountain. 
The held work was done by Mr. T. Nelson Dale, assisted in part of the 
area by Mr. William H. Hobbs. The analysis of the results and the pre- 
sentation are by Mr. Dale. 

As during the first two years we had not yet the benefit of the new 
topographic map of Massachusetts, our work was delayed by the necessity 
of making our own maps. This was done in part by Messrs. Putnam and 
Wolff, assisted by Mr. Yocum. Later, Mr. Josiah Pierce made a detailed 
topographic survey of the western flank of Hoosac mountain which forms 
the geographic basis of PI. iv. 

Mr. C. L. Whittle was also connected with the work under Dr. Wolff 
during the season of 1887. 

Mr. William H. Hobbs acted as assistant to Mr. Dale during one season 
and a part of another in the work on Greylock and was engaged inde- 
pendently during the rest of the second season on the coloring of the 
northwestern part of the Greylock sheet. 

I have mentioned in its proper place the fact that we owe to Mr. C. D. 
Walcott the determination of the age of our basal quartzite. 

R. P. 



PA^RT I 



GENEIUL STRUCTURE AND CORRELATION. 



By RAPHAEL PTJMPELLY. 



MON XXIII 1 



CONTENTS. 



Page. 

General description : 5 

Age ami stnieture - 7 

Correlation 9 



ILLUSTRATIONS, 



Page 

Pl. I. Map of Greylock and Hoosac Mountains Frontispiece. 

11. General map showing relation of the Greylock series to the Hoosac. mountain rocks 10 

III. Structural relations of the Hoosac series - 14 

Fig. 1. The Stamford dike, showing Caoibrian conglomerate deposited in dike fissure 11 

2. The Stajnford dike, plan 11 

S. Correlated columns of the Hoosac and Greylock rocks , . 13 

4. Anticlinal arch across Hoosic river between North Adams and Briggsville 15 

5. Ideal sectiou east of Cheshire, showing lateral transition of limestone to schist 17 

6. Diagram of structure, summit of the buttress 22 

7. Crumpled structure in albite-schist 23 

8. Map showing the varying character of Cambrian rocks around the Hoosac core 31 

3 



GEOLOGY OF THE GREEN MOUNTAINS IN MASSACHUSETTS. 

GENERAL STRUCTURE AND CORRELATION. 



By Raphael Pumpelly 



GENERAL DESCRIPTION. 



The Green mountains, nearly coinciding with the prolongation of the 
axis of the Archean core of the Appalachians through western New Eng- 
land, stand between the less disturbed fossiliferous Paleozoic strata of New 
York and the highly crystalline rocks of New England. They consist of 
three principal structural elements : The Green mountains (Hoosac moun- 
tain) ; the Taconic range, lying several miles to the west ; and, between 
these, the great valley. But the whole region between the Hudson and the 
Connecticut has very properly been placed by Dana in one mountain sys- 
tem. I shall therefore follow Dana and distinguish between a central or 
axial ridge, flanked by an eastern belt extending to the Connecticut, and a 
western belt extending to the Hudson, though what I shall have to say refers 
mainly to the central belt and the neighboring portion of the western belt. 

The Green mountain range is composed of crystalline schists, which 
our results show to be of Cambrian and Lower Silurian age, resting on pre- 
Cambrian rocks, and it was long ago shown by Edward Hitchcock to have 
an anticlinal structure. The Avestern edge of this axial range is, for long 
stretches, marked by a loft}^ brow of quartzite, and for this reason the 
mountains present a very steep flank on the west. At the base of this 
western flank lies what is known as the valley of Vermont or, in Massachu- 



6 (iREKN MOUNTAINS IN MASSACHITSETTS. 

setts, the Berkshire valley. This valley has a floor of crystalline limestone, 
often a saccharoidal marble of Cambrian and Lower Silurian age, on which 
stand long island-like ridges of schist, of Lower Sihman age, and it extends 
with a breadth of several miles from northern Vermont to Alabama. The 
schist is everywhere underlain by the limestone, which is marked by 
the fertility of its soil ; and, along its whole length, its wealth of limonite 
ores has for more than a century formed the basis of important iron indus- 
tries. Li the folded strata of this valley belt in Vermont and Massachusetts, 
subsequent erosion has left island-like mountains, sometimes of anticlinal, 
but generally of svncliual structure, with more or less pitch in their axes. 
Instances of the latter are Eolus (Dorset), Anthony, Greylock, Everett, 
etc., rising to 1,500 or 3,000 feet above the valley, and surrounded to a 
greater or less height above the base by the limestone, and hea^^ly capped 
with the weather-resisting schist. Instances of anticlinal structure are the 
less elevated pine hill near Rutland and the ridge which connects it with 
Danby hill in Vermont. 

On the west, this limestone valley has for a wall the Taconic moun- 
tains, with peaks rising 1,500 to 2,500 feet above the valley. This is a 
synclinal range of the same Lower Silurian schist, but, having its trough 
at a lower level, the limestone foundation appears only at the base. 

Turning now to the region east of the axis of the Green mountain 
anticline, we find no great and continuous depression comparable to that of 
the valley of Vermont until we reach the Connecticut valley ; and this is 
occupied by much later strata — Triassic resting on Devonian. This eastern 
region is a very roughl}' mountainous mass of schist, and, though of plateau 
origin, is crossed by deeply cut transverse valleys, which receive longi- 
tudinal tributaries, whose courses are determined in the main by the 
geologic structure of the territorj^. All along the eastern edge of the axial 
belt of the mountains there occur such narrow, longitudinal valleys, and 
as they contain, more or less continuously, beds of limestone of either 
Cambrian or Lower Silurian age, they define the eastern limit of the Green 
mountain range proper, with less topographic but with equal geologic 
sharpness. 



GENERAL STliUCTURE AND (JORKBLATION. ^ 

AGE AND STRUCTURE. 

In Ijeginniug work on the geology of New England, two facts were 
apparent — that from the Grreen mountains eastward the rocks were all highly 
metamorphosed and crystalline; and that onlj" in two or three localities had 
fossils been found, and in these places the rocks were so much disturbed 
that it seemed hopeless to use them as starting points for the general work. 
I became convinced that our hopes of determining the age of the New 
England rocks lay in using the Green mountains as a bridge. In following 
this plan we were immediately met by the fact that on the main ridge — our 
proposed bridge — the rocks are not only highly metamorphosed and their 
structure the reverse of simple, but that the western edge of the ridge marks 
an abrupt lithologic change between the character of the rocks of the 
mountain and those of the valley, with the exception of the younger schists, 
which in places cap both the axial range and the valley hills. On the west 
the great limestone and an underlying great quartzite come eastward to the 
base of the mountain, while a careful reconnaissance showed no trace of these 
rocks as such upon the mountain, nor of such a combination on the eastern 
side. 

This difficulty, which met the earlier surveys, had led to various 
hypotheses in which faults and overturns played an important part. And 
while* the rocks of this main ridge were assigned by different eminent 
geologists to ages ranging from the Sillery^ to Huronian and Laurentian,^ 
the residuum of opinion has been of late in favor of Archean or at least 
pre-Cambrian age. The problem was undoubtedly too difficult to be 
solved without more ample means than were at the disposal of our pre- 
decessors. 

It was evident that our first and hardest work would be to find the key 
to the structure of the range. For this purpose I sought a region where the 
western edge should present, instead of a straight line, as many bay-like 
curves as possible, and where the structure of the ridge itself should show 
folds with pitching axes. I hoped in such a region to eliminate the difficul- 



' Logan colors them as Sillery on the Geological map of Canada, 1866. 

" C. H. Hitchcock : geological sections across New Hampshire and Vermont. Bull. Am. Mus. 
Nat. Hist., vol. I, New York, 1884 



8 GREE:sr MOUNTAINS IN MASSACHUSETTS. 

ties introduced by possible faults, as well as the temptation to infer their 
existence; and also in case of pitching folds to get, through radiating cross 
sections, a knowledge of the true order of bedding. 

These conditions were found well presented in the northwestern corner 
of Massachusetts. Here the western edg-e of the main ridg-e coraino- down 
from Vermcfnt makes a sharp turn eastward around Clarksburg mountain; 
then after resuming for several miles a straight southerlj^ course it curves 
back westward to bend around the Dalton hills. Opposite this bay stands 
Greylock mountain, which Emmons and Dana had shown to be a great 
synclinal mass. The greater and higher part of this Greylock mass of 
Lower Silurian rocks rises to the east of the chord of the arc that is formed 
by this bay-like curve. Again, Hoosac mountain, east of this bay, exhibits 
a variety of distinct rocks in folds, the axes of which show a persistent 
northerly pitch. And in addition to this I hoped for much aid from the 
great tunnel, which, in 1865, I had examined for the state of Massachu- 
setts. With a length of nearly 5 miles, it pierces the mountain through its 
whole breadth at a depth of over 1,000 feet, and the fact that the tunnel 
was driven from both ends and from two intermediate shafts gave assurance 
that the dumps would supply unaltered material for the petrographic study 
of the various rocks in all their variation of habit. As there Avas then no 
topographic map of the i-egion we were obliged to locate all of our work 
by transit survey. During the first two seasons, in company with my 
assistants, Mr. B..T. Putnam and Mr. J. E. Wolff, I made thorough recon- 
naissances of the area in question, and, to obtain as much light as possible, 
these excursions were extended southward to the Highlaiids east of the 
Hudson and northward to centi-al Vermont. 

We had found that the mass of Hoosac mountain consists of a core of 
coarsely crystalline granitoid gneiss, overlain in some places by a conglom- 
erate, in others by line grained, white gneisses. Above the conglomerate 
and white gneisses Ave had found a great thickness of biotitic and sericitic 
schists, containing either macroscopic or microscopic albite, in both un- 
twinned and simple twinned crj^stals. At all the contacts of this whole 
series there appeared distinct structural conformability. 

On Clarksburg mountain Ave had found the same coarse granitoid gneiss, 



GENERAL STEUCTURE AND CORRELATION. 9 

covered, apparently conformably, by a true quartzite. At the base of the 
Dalton hills the quartzite Avas found to conformably underlie the great 
Cambro-Silurian limestone, wliich in its turn forms the base of Greylock, 
and this. limestone was found to Be conformably overlain on Greylock by 
a great thickness of scliists, identical in character with those overlying* on 
Hoosac — here the conglomerate and there the white gneiss — with no inter- 
vening limestone or quartzite. Again, we had found that these white 
gneisses contained apparently iuterstratified beds of these same schists. 

CORRELATION. 

Having made it a rule that all correlation of strata and interpretation 
of structure should be decided solely upon observed structural relations, 
there was nothing to be done but patiently to work out the structiu'e, step 
by step, using lithologic similarities as clews only. 

The reconnaissances showed that the Green mountains are wholly 
made up of crystalline schists, and that one or more of the horizons of 
these must vary in the most jjrotean manner in the external habit of its 
rocks, while on either side of the range' the rocks retain their respective 
characteristics with relatively little change. One of the earlier observa- 
tions on the western brow of Hoosac mountain had been the superposition 
of the coarse granitoid gneiss over the white g-neiss at a well-marked con- 
tact and with structural conformity of lamination. On the other hand, in 
the tunnel, this same granitoid gneiss appeared as a central core, fiirtiier 
east than the geologic meridian of the surface outcrop. This core Avas 
found in the tunneP to be flanked on each side by the conglomerate over- 
lain by the albitic schist. If the structure were as simple as the tunnel 
section seemed to indicate it would point to two horizons of the granitoid 
gneiss, and connect this rock and the white gneiss in age. An important 

' Diuui pointed out iu 1872 tlie abrupt lateral transitions between the quartzite and schists of 
Berkshire county. (Ami. Jour. .Sci., 1872, p. 368.) 

2 This tunnel is lined with masoury at irregular intervals to such an extent that a large part of 
the rock, especially of the more interesting western half, is hidden. The walls are covered to a depth 
of an inch with soot. In addition to this, geologic work was made extremely dangerous by the fact 
that the smoke was so dense that even our thirteen torches were invisible across the tunnel, and the 
noise of trains running 30 miles an hour was not audible until the engine was within a few yards from 
us. Notwithstanding these difficulties we managed to find the important contacts, except at the 
■western end, where they were bricked over. 



10 GREEN MOUNTAINS IN MASSAOHUSETTS. 

point was therefore gained when the hypothesis advanced to us by Mr. 
Putnam that the surface exposure of granitoid gneiss was a flat, overturned 
anticlinal fold was corroborated by Mr. Wolff. Mr. Wolff also discovered 
that the schist beds in the white g^neiss on the western flank belong to the 
series above the wliite gneiss, and are simply remnants left in compressed 
troughs overturned to the west under the overtmnied anticline just men- 
tioned. 

The next step was made by Mr. Wolff in the determination that the 
white g-neisses are clastic rocks, while the coarse granitoid gneiss shows no 
trace of clastic origin. This pointed to a closer relation between the white 
gneiss and the conglomerate, from the fact that one or the other was found 
to overlie the granitoid gneiss. This question also was settled by Mr. Wolff 
by tracing out the lateral transition from the conglomerate into the white 
gneiss. 

Finallv the upward transition from the white gneiss and from the con- 
glomerate into the schist was observed. 

Messrs. Putnam and Wolff had observed, and I had traced later at several 
points on Clarksburg mountain, a strict conformability between the lamina- 
tion of the granitoid gneiss and that of the overlying conglomerate and 
quartzite, the continuation of the great quartzite belt of Vermont ; and later 
Mr. Walcott had found, near the same contact, numerous casts of OlcneUus, 
showing the lower part of the quartzite to be of Low^er Cambrian age. Later, 
Mr. Wolff, in tracing this quartzite northward along the eastern flank of the 
granitoid gneiss of Clarksliurg mountain, found it to pass by lateral transi- 
tion along the strike into well-defined white gneisses like those of Hoosac 
mountain. Later still a similar transition was observed between the true 
quartzite and the Hoosac white gneiss on the northern side of the Dalton 
hills. 

There still remained to be explained the nature of the relation between 
the granitoid gneiss and the overlying clastic rocks, and the conformability 
that exists between the stnicture of the granitoid and that of the overljang 
rocks. Prof. Emerson, working on the map in Hinsdale, found an area of 
granitoid gneiss overlain by the conglomerate, and concluded, from the re- 
lation of the two rocks over broad areas, that they are there structurally 



U.S.GEOLOGrCAL SURVEY: 



MONOGRAPH XXII PL II- 




SHOWING THE RELATION OF THE GRFiXOCK SERIES TO THE HOOSAC MOUNTMN ROCKS. 

BY RAPHAEL PUMPELLY. 



Scale, 125000 



=iMiles 



1891. 



GENERAL STRUCTURE AND CORRELATION. 



11 



unconformable. At about the same time Mr. Wolff had found the two dikes 
of eruptive basic rock in Stamford in the g-ranitoid gneiss and at its contact 
with the quartzite. (Figs. 1 and 2.) 



J? 'i 




N 

A 



Fio. 1. — The Stamford dike, showing the Cambrian conglomerate deposited in dike fissure; (7, 
conj;l"iiifriite; c. l(^^ve^ layers of conglomerate rendered schistose by admixture of material from 
the altrre<l dike; d, diah:ise of the dike rendered schistose hy metamor]ihisni : p. nltercil dike ma- 
terial ; */. pre-Ciiiubrian ;;Taiiitoid jrueiss. 

We could hardly have wished for better evidence than that offered by 
one of these. At the contact the quartzite strikes N. 40° E., dips 50° SE. 
The dike strikes N. 60° W., between vertical Avails; but the rock of the dike 
has undergone changes that have given it a lamination, and the planes of 
this strike N. 35° W. and dip 45° eastei'ly. The structure of the granitoid 
gneiss is here quite irregular and obscure. No trace could be found of the 
dike cutting into the quartzite, and as 
this is continuou.sly exposed on both 
sides, the possibility of its absence by 
faulting was eliminated. But there is 
more direct positive evidence in the fact 
that the quartzite beds thicken and sag 
down over the dike — indeed, into the 
dike fissure, as Mr. Whittle and I found 
by digging. The evidence is conclu- 
sive, as I satisfied myself during several 
visits, that the Cambrian transgression 
found here a fissure, either open or filled 
with a rotten dike, which was washed 
out to a depth of several feet and refilled with beach sand and pebbles, 
the dark material contributed by the dike increasing toward the bottom. 
The sudden thickening and sagging of the quartzite over the fissure, taken 




Fig. 2. — The Stamford dike, plan, c, conglomerate; 
d, dike rock, metamorphic. with foliation; e, altered 
dike material; g, Stamford gneiss. 



12 GREEN MOUNT AmS IN MASSACHUSETTS. 

in connection with the mixture of dike material and sand, and the stoppmg 
of the dike at the quartzite, prove suificieutly the pre-Cambrian age of the 
granitoid gneiss. And this is emphasized by the fact that both the quart- 
zite and white gneiss are frequently conglomerates. 

The structural conformability of which I have spoken above is due 
simply to the generally parallel lamination that has been forced upon the 
rocks of the region by the folding. 

We had now established the fact that in this part of the Green mountains 
the eolunm in the main range consists of a Lower Cambrian quartzite-con- 
glomerate-white-gneiss formation, resting with a time break ujDon a coarse 
granitoid gneiss, and conformably overlain by a great thickness of schists. 

Parallel with the study of Hoosac mountain, that of Greylock was carried 
on by Mr. T. Nelson Dale, assisted, for a time, by Mr. W. H. Hobbs. This 
mass was shown to consist of a great lower crystalline limestone, overlain 
by a heavy mass of schists, above which another thick mass of limestone 
was overlain hj still another great mass of schist, the whole column contain- 
ing about 2,000 feet of limestone, and 2,500 to 4,000 feet of schist. These 
estimates are based on measui'ements of areas that have been subjected 
to lateral pressure, and of course do not claim to represent the original 
thickness. 

Lower Silurian fossils have been found in the continuation of a part of 
the lower limestone in Vermont. Mr. Dale found the Greylock limestone 
and schists conformable throughout and exhibiting vertical transitions. 

It seemed almost impossible to find points where the actual stratigraphic 
relation of the limestone to the quartzite could be observed, but I was for- 
tunate in finding such a place on Lachines creek, near Berkshire station. 
Later, by means of digging, which was done here under Mr. Putnam, it was 
shown not only that the quartzite and limestone are structurally conform- 
able, but that they are bound together by vertical transition through calca- 
reous flaggy quartzites. We have here in an overturned fold, with easterly 
dip, the Stockbridge limestone dipping under the older Cambrian quartzite 
formation. The limestone proper is succeeded toward the quartzite by 
flaggj?^ quartz schists, and these by a heavy development of schistose calca- 
reojas quartzite. East of this the quartzite becomes friable, and has here 



GENEEAL STKUCTUKE AN1> COREELATIOK 



13 



been excavated as the well-kuown Berkshire sand. About 100 feet east of 
this we find an outcrop of vitreous qiuirtzite. The next outcrops — okler 
and 300 or 400 feet eastward and dipping 50° easterly — show a schistose 
quartzite overlain by an older and more slaty bed of the same; and this by 
a very coarsely feldspathic quartzite followed by another bed of schistose 
quartzite, and this by a feldspathic biotite-schist. Representing these in 
their normal succession we have — 

Stockbridge liiuestoue. 
Flaggy quartz-schists. 
Schistose calcareous quartzite. 
, Saudy (juartzite (friable). 

Vitreous quartzite. 
Covered (300 or 400 feet). 
Schistose quartzite. 
Schistose quartzite, more slaty. 
Very feldspathic quartzite. 
Schistose quartzite. 
Feldspathic biotite-schist. 

HoosacMt. 



GrvjlockMt. 



Mclloyvspipc 2jtm£S6oTW , 
Sei^k.?}Ltre Sch LsL 



f^rtrr-/;,-!--: 






Howe 'ScfUst. 



Ifoosfxc Schist, 



Quiu-tzUeConfflomerate., 
Sfajri/oT'cL Gneuf^. 



Fig. 3.— Correlated columns of the Hooa 'C and Greylock rocks. 

We now had both the Hoosac and Greylock columns complete, and 
both springing from the same conformably underlying Cambrian quartzite 
(see Fig. 3). 

A glance shows one point of difference — the entire absence of limestone 
in the Hoosac column. But on the other hand, we have the observed con- 



14 GREEN MOUNTAINS IN MASSACHUSETTS. 

tiuuity of deposition from the quartzite upwai'd in each column, and we 
have also petrographic identity in the schists of the two columns. 

Prof Emmons attempted to explain the similarity of the Greylock 
schists to those of Hoosac mountain by deriving the supposedly younger 
Greylock beds from the destruction of the supposedly older Hoosac rocks, 
but Mr. Wolff finds, under the microscope, not only no evidence to sup- 
port the idea of such derivation for the Greylock schists, but that the 
principal constituent minerals of these schists were in each column all 
crystallized in place. Early in the course of the work it was proved that 
the limestone was not present as such in the Hoosac column. But at 
two points near Cheshire harbor, and east of North Adams, we found schist 
outliers. extending out from the Hoosac column, and at the extreme western 
ends conformably related to the great limestone; in one case occupying a 
' synclinal trough in it, and in the other either capping it or interbedded in it. 

Almost at the beginning of the survey, altliough we had as yet none 
of the proofs above given as to the equivalence of the valley quartzite with 
the Hoosac conglomerate and white gneiss, the strong possibility that at 
least a part of the Greylock column was contemporaneous with a part of the 
Hoosac column had presented itself to me. This possibility was strength- 
ened when we had correlated the quartzite with the white gneiss and con- 
glomerate beds as equivalents. The truth of this hypothesis could be tested 
only by finding beds showing lateral transition to bridge the narrow belt 
between the Stockbridge limestone and the Hoosac schist. 

In the progress of om- survey we found, at various points between the 
valley and the mountain, and always east of the limestone, outcrops of a 
peculiar rotten schist — quartz and mica with some feldspar, with the mica 
arranged in long narr(jw flakes and with sufficient calcite to show the cause 
of the decomposition. The occurrence of this peculiar calcareous rock 
along the boundary between limestone and quartzite, as on Tophet creek 
and below the albitic schist in the western end of the tunnel, shows that it 
belongs in the horizon of the vertical transition between the quartzite and 
the limestone, and it seems to represent also the lateral transition zone in 
this horizon Ijetween the Hoosac and Greylock columns. 

East of North Adams, on the road to Briggsville, the river cuts longitu- 








in 




JZ 

to 




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1*3 


■OC 














a 


to 





ca 

►J 
<! 
o 

o 
o 

o 

o 

CO 

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S 



7X. 





^ 










^ 

^ 


,^ 


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c 


^1> 


«> 


(1 


■t) 






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g 5 M'l'. ' 




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J3 SYiiliii'iIi^l 


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^§ Wi'"",' 




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J-ijA 











GENERAL STEUGTURE AND CORRELATION. 15 

diually through an anticline ; a few huu tired feet west of the river there is 
a massive anticline of marble exposed in large quarries ; the eastern end 
dips toward the river, but a sharp anticlinal fold, slightly overturned to the 
west, brings the strata, up near the west bank of the stream, in interstrati- 
lied beds of limestone and schist. The arcli springs over the river, and its 
easterly dipping limlj forms a high cliff on the eastern bank. In this eastern 
limb the limestone is represented by calcareous siliceous-micaceous schists 
and very impure limestones. The whole arch is exposed near by, in a cliff 
in the bend of the river (see Fig. 4). 

This is the most eastern exposure of limestone, and there can be no 
doubt that we are here in the zone of lateral transition between the condi- 
tions that produced in the same horizon the Stockbridge limestone and part 
of the Hoosac schist. Again, along the north base of the Dalton hills, in 




Fig. 4 — Amiclinal arch across Hoosic river between North Adams and Briggsville, 
ill the zoiie of lateral transition hetween Stockbridge limestone and Uoosac scliist: 
a, limestone more or less micaceous and siliceous; 6, calcareous and siliceous sithist 
witll thin layers of limestone; an, interstratified siliceous and micaceous Jimestonc, 
calcareous quartzite and mica-schist; 66. less calcareous garnetiferous schist. 

Cheshire, Mr. Wolff found a schist consisting of calcite, mica, quartz and 
simple twinned albite, which, from its position and nature, vmdoubtedly 
represents this zone of lateral transition from limestone to schist. 

If the reader will turn to Plate ii he will see that the Stockbridge 
limestone sends a broad rectangular bay southeast in Cheshire to conform 
to the embayed topography of the Dalton- Windsor hills. In the middle of 
this embayment he will observe a detached area of Berkshire schist of an 
irregular sha[)e, suggesting a long-eared rabbit. There is no question as to 
the continuity of the schist over the area as represented. The long rabbit- 
ear-like area lies upon the limestone in a synclinal trough. The structure 
of this area is not simple ; it is that of a small synclinorium, the axes of the 
north-south running folds pitching toward the center, and the folds at the 
northern end being more or less overturned to the west in conformity with 



16 GREEN MOUNTAINS IN MASSACHUSETTS. 

the general overfolding of Hoosac mountaiu and the Daltou-Wiudsor hills. 
Tlie limestone proper borders the whole western side of the area and ex- 
tends well into the bay east of Cheshire. On the east side it also extends 
visibly down from the north for some distance, but it then disapjjears under 
a heavily drift-covered area. Going south from the limestone on this east 
side, the first exposures we find belong to a continuous belt of the schists 
connecting the Cheshire schist area with the tongue of schist infolded in 
the Cambrian white gneiss farther east at the base of Hoosac mountain. 
There is neither any trace of the limestone nor any room for it. 

On the south of the Cheshire schist area the Cambrian quartzite covers 
the Dalton- Windsor hills, the topography of which is formed by the undu- 
lations and intervening sharp folds of this hard mantle. The dip of the 
undulating quartzite beds and the pitch of their sharp folds are both toward 
the center of the Cheshire schist synclinorium. 

The Cheshire schist hills are separated from the higher Dalton- Windsor 
quartzite hills by a narrow valley, which curves around the southern end of 
the former with few exposures. But at one point quartzite and schist are 
very near together, and it is evident that there is no room: for the limestone 
as such. In this valley there are large numbers of gr.eat angular blocks 
and at least one ledge belonging to a transitional scldst formation. I 
repeat here L)r. Wolff's description of this important rock: 

It resembles a micaceous white limestoue fiUed witli little dark grains or imper- 
fect crystals of feldspar. Under the miscroscope, in thin section, it is composed of a 
mass of calcite grains, with here and there single grains of quartz, or an aggregate of 
several grains, plates of muscovite and often of chlorite and biotite, and large por- 
phyritic feldspar grains in single crystals or simple twins, very rarely showing poly- 
synthetic twinning. These feldspars contain inclusions of mica, quartz, iron ore, 
rutile, and calcite. and are in every way identical with the albites of the albitic 
schists, although the exact species of plagioclase has not been determined. The 
calcite seems to play the part which the quartz does in the schists: it sends tongues 
into the feldspars or cuts them in two, and gives one the impression by its inclusions in 
the feldspar, and its occurrence with the quartz and mica, that it is of contempora- 
neous origin with the feldspar, mica, and quartz. 

This schist represents the landward transition of the Stockbridge lime- 
stone into the Hoosac albitic schist. Thus the Cheshire schist area is at its 



GENERAL STRUCTURE AND CORRELATION. 



17 




Fig. 5. — Ideal section east of Cheshire, showing 
lateral transition of limestone to Hoosac schist; S, 
Berkshire schist; i, Stockbridge limestone: ^, Lower 
Cambrian qnartzite of Dalton- Windsor liills; Ci/, cal- 
careous quartzite: transition quartzite to limestone; 
OS, calcareous feldspathic scliist in lateral transition 
from Stockbridge limestone to Hoosac schist. 



northern end simply the Berkshire schist resting upon the Stockbridge 
limestone, while as we go southward we find it representing not <^nly the 
Berkshire schist, but also the whole thickness of the limestone itself, and as 
we go eastward we find through continuous exposures its connection and 
identity with the tongues of schist infolded in the Cambrian quartzite 
gneiss of Hoosac mountain. 

In Fig. 5 I have attempted to represent, in a somewhat ideal section, 
the transition from limestone to schist at the south end of the Cheshire hill. 
The transition is clearly quite abrupt, 
and might easily occur within the space 
represented by the eroded folded arch 
between the limestone and the infolded 
schist along the west base of Hoosac 
mountain. See c, PI. iii. 

The western end of the Hoosac tun- 
nel lies in the belt of this lateral transi- 
tion of the Stockbridge limestone into the 
Hoosac schists; but it is now completely hidden by the brick arching ren- 
dered necessary by the decomposed condition of the material. Indeed, it 
acted for several hundred yards from the portal as a quicksand, and the 
tunneling work had to be preceded by small tunnels incased in closely 
matched planks, so fluid was the decomposed water- saturated rock. I have 
attempted to represent the structural facts at this point on the west flank of 
Hoosac mountain in D, PI. iii. 

At the time of my examination of the tunnel, in 186.5, the limestone 
was exposed in open cuts and tunnels — nearly parallel to the present open 
cut — ^for nearly 700 feet east and west. The exposure showed in this dis- 
tance two rather flat anticlines. The eastern limb of the easternmost anti- 
cline dipped east and was for a short distance concealed by masonry. East 
of this was an open cut, for nearly 400 feet, in the decomposed rotten schist, 
which seemed to show faintly preserved indications of an easterly dip. 
Just east of the middle of the cut a less altered bed showed a well-defined 
syncline with an anticline on the east and having the eastern limb of the 
latter exposed in the heading with easterly dipping structure. 



MON XXIIT- 



18 GREEN MOUNTAINS IN MASSACHUSETTS. 

From the above description it will be seen that the actual nature of 
the relation of the limestone to the rotten schist was hidden. But just west 
of where the contact should be I found the limestone conformably overlain 
by a few feet of Hoosac schist. Farther east is a small shaft, from which 
was hoisted some of the rock excavated between the western headings of 
the "west" shaft and the open cut; this rock is a more or less rotten calca- 
reous feldspathic mica-schist, having the same elongated structure parallel 
to the axes of the folds as in the rotten transition schists of this zone, and 
marked by the same similarly an-anged long, narrow flakes of mica. It 
recalls in structure, also, at once, the calcareous gneiss associated with the 
limestone on its eastern border near South Adams, and also the noncalca- 
reous and rather less feldspathic mica-schist of the "Buttress" core. I 
think that, taken in coimection with the facts observed south and east of 
Cheshire hill, we have in this rock the upward transition from the quartzite 
to the limestone brought to the tunnel line in an anticlinal arch, and that 
we have, in the wholly decomposed material of the former open cut, the 
lateral transition from the rest of the limestone into the Hoosac schist. A 
few hundred feet, from east to west, would span the whole lateral passage 
from limestone to Hoosac schist. This transitional calcareous schist decom- 
poses much more easily than the limestone and is therefore more rarely 
seen. Nevertheless, as stated above, it is found exactly where it should 
occur as such a transitional form, not only in the western end of the great 
tunnel, but at several points along the western base of Hoosac mountain 
above the quartzite and west of the infolded schists. 

While the rocks of the zone of lateral transition, in the horizon of ver- 
tical transition from quartzite to limestone, were tolerably hard, they suc- 
cumbed to disintegrating agents much quicker than the quartzite proper. 
But the rocks of the zone of lateral transition between the limestone and 
Hoosac schist, being calcareous schists, were adapted to the most rapid 
destruction, and we therefore find them only where the conditions for their 
preservation have been exceptional. 

From Cheshire hill northward this zone covered anticlinal folds turned 
over to the west, which have been to a great extent eroded down to the 
harder beds towards the true quartzite. It does not seem improbable that 



GENERAL STEUCTUEE AND COEEELATION. 19 

the zone of lateral transition of limestone to Hoosac schist was a zone of 
weakness which had ranch to do with the overfolding along the west base 
of Hoosac mountain. These anticlinal axes are inclined gently to the 
north. About a mile south of the tunnel, at the "Buttress," the core of one 
is visible as a hard, white gneiss, but at the tunnel line it has sunken to 
where the erosion surface cuts the beds representing the lateral transition 
of limestone to schist, where they mantle around the pitching anticline, 
and before they disappear under the younger schist beds which stretch 
out from the mountain. 

While the equivalence of the Greylock column with a large part of 
the Hoosac column can be thus asserted, I am not yet in a position to make 
a correlation reaching into details. It is not possible with our present data 
to subdivide the Hoosac column into equivalents of the two schists and two 
limestone horizons of Greylock. There is, indeed, in the eastei'u half of 
the Hoosac mountains a rather sharply defined plane of division, separating 
the feldspathic schist on the west from the practicall)^ nonfeldspathic schists 
on the east, and these latter are distinguished further by the fact that their 
quartz is distributed in thin, even layers, instead of occupying lenses, as in 
the rocks to the west. This plane is used by Prof Emerson as the base 
of his lower hydromica-schist, and forms an impoi-tant horizon of reference 
in his work east of the mountain. The thickness of the albitic schists 
between this plane and the conglomerate has not yet been determined, 
as the structure is masked by the cleavage. It is certainly not more than 
5,600 feet, and probably not less than 2,500 feet. If there are no faults 
or foldings, it is probably about 4,000 feet. We are equally ignorant of 
the real thickness of the Greylock beds, after allowing for the effect of 
lateral pi'essure and increasing local thickness. But it is quite possible, 
if not probable, that these nonfeldspathic schists belong wholly above the 
Greylock rocks. In the study of Greylock mountain Mr. Dale, by patient 
search for the traces of the original stratification, which have here and there 
escaped the general obliteration caused by cleavage, has been able to work 
out the details of surface structure quite closely, and to obtain a general 
idea at least as to the maximum thickness of the two limestones and two 
schists. But the compressed foldings have so altered the thickness of the 



20 GREEN MOUNTAINS IN MASSACHUSETTS. 

strata that it is impossible to give the real vertical dimensions. His 
estimate is: 

Greylock schist 1, 500-2, 000 

Bellowspipe Umestoue 600- 700 

Berkshire schist 1, 000-2, 000 

Stockbridge limestone 1, 200-1, 400 

These are, however, based on measurements of beds that have been 
subjected to strong lateral compression, and, as Mr. Dale observes, although 
the aggregate maximum of the thickness given above is below that assigned 
to the Lower Silurian in the Appalachian region, it is probably far in excess 
of the real thickness, which maybe considerably below the maximum above 
given. 

^ The sediments which in vast thickness form the substance of the Grreen 
mO'Untain system have been subjected to intense lateral thrust, which has 
produced numerous folds. These, as a rule, are more or less compressed 
and overturned to the west, in places indeed forced over until the axial 
plane lies almost horizontally, or compensations have taken place through 
overfaulting. The sections and map of the Hoosac-Grreylock ret,ion illus- 
trate the structure in its generality. 

From these it will be seen that on Hoosac mountain the granitoid 
gneiss and the overlying conglomerate gneiss-quartzite and albitic schists 
have been folded into a low anticlinal arch, the western side of which has 
been forced over to form an overfold to the west. 

An examination of the longitudinal sections on Plate vi accompanying 
Part 11 (Mr. Wolff's report) shows that the southern end of this arch is over- 
folded in the same manner, but to the south. We have thus the remarkable 
occurrence of an overturned anticline abruptly turning a right angle. A 
glance at the map (Plate ii) will show that this is repeated by the next over- 
folded anticline to the west, which bends equally abruptly around to run 
eastward, and that the inverted trough between these anticlines is still 
marked by the infolded band of schist. Groing from this southward, Ave 
come immediately upon another east and west trough of schist, also over- 
turned to the south. Still further southwest, we find along the northern 
part of the Dalton- Windsor hills the quartzite gneiss beds thrown into 



GENERAL STEUOrUEE AND COREELATION. 21 

overfolds, but witli the axes striking northwest to southeast ; while still 
farther westward they are overfolded to the west, but with the axes in the 
normal position of the Grreen mountain folds — nearly north and south. 

Looking at the map and sections of Grreylock, Pis. i, xviii, xxiii, we find 
a great basin-bottomed mass, thrown into numerous more or less overturned 
folds, with axes in the normal Green mountain position, and inclined from 
each end toward the middle. Again, if we look at the eastern border of the 
map, we find in the observed strikes and dips of the conglomerate gneiss and 
schist east of the granitoid, no trace of a departure from the general Green 
mountain direction. 

This local modification in the structure of Hoosac mountain must be 
due to some local cause, which I think must be sought in the pre-Cambrian 
topography. The Greylock basin of sediment was guarded on the north 
by the large mass of granitoid gneiss of Clarksburg mountain, and on the 
south by the great body of pre-Cambrian rocks which are now masked by 
the Dalton and Windsor quartzite. I imagine that the lateral thrust to 
which the foldings are due 'met with greater resistance opposite these mtire 
rigid granitic masses than in the interval, and that the abnormal overfoldings 
to the south, described above, are the result of compensatory movement. 
The Hoosac mountain cross sections show a much more marked overturn 
than is observed to either the east or west of it. The axial plane of the 
principal overturned fold on the west side of the mountain lies very flat. 
We may suppose the greater rigidity of the granitoid gneiss ' to have 
caused it to yield as a unit to the contracting force. Only its relatively 
nari'ow top participated in the actual folding and was carried over to form, 
with the leeward, protected beds, a flat-lying, compressed syncline. 

A similar overturn, though not so flat, was observed by us on Sumner 
mountain, in Pownal, on the west of the Clarksburg mass of granitoid 
gneiss. Section a on Plate iii was made by Mr. B. T. Putnam. - I have 
added my interjjretation in dotted lines. This outlier is separated from 
Clarksburg mountain by Broad brook, this interval being occupied by 
the quartzite. The large Clarksburg mass of granitoid gneiss remained 
a dome mantled by the Cambrian quartzite, and showing the effect of the 
folding force only in the induced lamination common to itself and the 



22 • GREEN MOUNTAINS IN MASSACHUSETTS. 

quartzite, while in the smaller mountain to the west, which has a grani- 
toid gneiss core, this core is pushed up in the form of an overturned anti- 
cline upon which the quartzite lies, in normal position on the east, while on 
the west the granitoid is underlain, in inverted order, by the quartzite and 
the limestone. 

A careful study of the western flank of Hoosac mountain shows that 
its structure is not that of a simple, great, overtm-ned fold. It consists of a 
series of parallel, crumpled folds, one or more of which have a greater depth 
than the others. All of them are overfolded, with their axial planes dip- 
ping eastward and with their axes pitching about 10° northward. The 
average chord-plane of these folds dips westward 15° to 20°, forming thus, 
as a whole, a comparatively flat, though much crumpled, western limb of 



Fig. 6.— Diagram of structiu'e, summit of the Buttress, ou west flank of Hoosac 
mountain, about one mile south of Hoosac tunnel, a, Buttress rock, upper part .» 

of Cambriau white gneiss ; b, Hoosac schist. The exposure at the east end is part 
of the long trough infolded along the whole front of Hoosac mountain. 

the main Green mountain anticlinal arch. This structure is shown in nu- 
merous preserved fold-cores, and is illustrated in the section through the 
"Buttress" (Plate in, c) and in the annexed diagram of the summit of the 
same hill (Fig. 6). The " Buttress"— a high hill on the flank of the mountain 
about one mile south of the tunnel— is the southerly extension of one of the 
larger of these crumples, where the axis in rising to the south brings up the 
harder core of Cambrian white gneiss. The structure is marked both by the 
preserved fold-core at a, just west of the summit (Fig. 6), and by the small 
infolded troughs of younger schist at b on the summit and h on the western 
flank. Further north, as at the tunnel line, where nearly the whole flank of 
the mountain is covered by the schist, the crumpling is much greater, as 
one would expect in this material, and is marked by the crumpled layers of 
quartz (Fig. 7). " Toward the south end of the mountain, near where the 



GENERAL STEUCTUEE AND COEEELATION. 



23 



great schist trough is seen on the map to turn sharply to the east, the evidence 
of this same structure is preserved in several minor iufoldings of schist. 

In the tunnel the rotten rock of the old open cut, and that which I 
have described as the Buttress-core rock and as forming below it the 
upward transition from quartzite horizon to limestone horizon, are con- 
cealed by masonry. But from a point several hundred feet west of the 
"west" shaft we find the Hoosac albitic schist, which extends some 1,400 
or 1,500 feet fui-ther east till we reach its contact with the underlying con- 
glomerate-white-gneiss (See PI. Ill, d). This last-mentioned rock extends 
some 2,000 feet farther east to its contact with the pre-Cambrian coarse 
crystalline gneiss of the Hoosac core. On both its eastern and western sides 



w 




Fig. 7 Crumpled structure in the Hoosac schist above the " west shaft" 

on Hoosac mountain, a, cleavage foliation ; b, stratification lines marked by 
crumpled quartz layers. 

the contact planes show that the Cambrian white gneiss is overturned in a 
flat-lying anticline. Leaving, now, the tunnel and climbing to the opening 
of the "west" shaft on the flank of the mountain we find that the upper 
part of the shaft is in the Buttress-core rock — quartzite-limestoue transi- 
tion rock — and that the same formation crops out upon the mountain until 
we reach the Hoosac schists, several hundred feet higher up. Climbing 
above this point we find the Hoosac schists, with evidence that they occupy 
an inverted syncline. Fig. 7 shows the structure at this point on a small 
scale. Above this the dips observed on both sides of the summit show that 
the crest is a simple open syncline. 

The presence of the Buttress-core rock at the top of the "west" shaft 
and its projection so far westward Qver the Hoosac schist of the tunnel 



24 GREEN MOUNTAINS IN MASSACHUSETTS. 

below can be explained only by introducing an overthi'ust fault or by sup- 
posing that the inverted anticline was pushed out thus far without rupture. 
The former explanation seems the more likely one and accords better with 
the thickness of the Cambrian gneiss and the dips in the schist observed in 
the tunnel. The bed of white gneiss— 600 to 800 feet thick— when ex- 
posed to the great thrust which overfolded the Hoosac rocks, would, it 
seems, be less able to adapt itself by minor foldings than the more readily 
yielding schist, and would be more likely to find its compensation in a 
rupture and an overthrust fault. 

At the tunnel line the axes of the folds are still pitching to the north. 
Immediately north of the limestone is a mass of folded Hoosac schist, under 
which the limestone is earned by the pitch of its folds and which is seen at 
several points to be younger than the limestone. The zone of lateral tran- 
sition is also Qan-ied under this hill, and this fact explains the peculiar areal 
geology of this part of the map (PI. i) on which the color for the Stock- 
bridge limestone extends along the west side of the schist hill, that for the 
Vermont formation along the east side. The obscurity disappears on PI. ii, 
where I have separated these transitional rocks from the quartzite and given 
to them and to the lower part of the limestone a separate representation as 
Cambrian. 

It is not easy to determine the extent to which overthrust faulting has 
entered into the building of the Grreen mountain range in northwestern Mas- 
sachusetts. Along the eastern side of the pre-Cambrian core of Hoosac 
mountain the movement flattened the coarse pebbles of the conglomerate 
and granulated their quartz and large feldspars to the point of obliteration. 
But the great Cambrian conglomerate-gneiss bed as it curves around the 
core shows no break due to faulting. It is not until we come to the west 
side of the pre-Cambrian gneiss-core that we find evidence of a rupture in 
the flat fold, where the hard Cambrian white gneiss has been pushed along 
an overthrust fault ont(i the younger schists as far as the west shaft. Here 
it seems, probable that the rupture was favored by the fact that the troughs 
of the folds, both above and below the middle limb, were on the lee side 
of the less yielding pre-Cambrian core, as will be seen from the section 
(PI III, u). Now this is the same fold that incloses the trough of schist all 



GENERAL STEUCTUKE AND COEEELATION. 25 

along the west side and south end of the mountain, and it is not impossible 
that it may be accompanied there, as here, by the same ruptvire. If this is 
so, then the position of this overthrust plane would lie above and to the east 
of the schist trough shown on the Buttress (PI. in, c, d). 

Having sketched thus briefly the general relation of the crystalline 
schists of the main ridge of the Green mountains to the fossiliferous rocks 
lying to the west, let us now return to the main ridge. 

We have seen that the Cambrian white gneiss rests with a time break on 
the coarse granitoid gneiss In places on Clarksburg mountain we find the 
micaceous quartzite more or less conglomeratic at the base, resting on the 
gi'anitoid gneiss, the two rocks sharply distinct. In others, as on Hoosac 
mountain, a conglomerate rests on the granitoid gneiss with sharp definition. 
But this simplicity is not always present, especially at the meeting of the 
white and granitoid gneisses. In general there intervenes between the 
well-defined coarse gneiss and the well-marked white gneiss a zone of beds 
of luore or less coarse gneiss, often alternating with finer grained biotite 
schists. It is not easy in such places to draw the line between the Cam- 
brian and pre-Cambrian formations, though, as I will show further on, in 
some instances there is good reason to draw the line at the base of the 
transitional beds where these show alternating strata of varying character. 
One thing appears certain : the dynamic action which has folded these rocks 
has impressed upon them not only their cleavage and plication, but also the 
I'emarkable simulation of conformity in bedding and of vertical transition. 

The pre-Cambriau core of the Grreen mountains reappears at frequent 
points along the range. In places it forms almost island-like masses of old, 
hard gneisses surrounded by the Cambrian quartzites and allied rocks, as 
in the northwestern corner of Connecticut. In others, as on Hoosac and 
Clarksburg mountains, it appears as limited, oval, dome-like areas of granitoid 
gneiss. Again, as in Chittenden, Vermont, it consists of a long, narrow 
line of coarse gneiss, at eroded points in the backbone of the range. 
Finally, as between Clarendon and Ludlow, in Vermont, where the height 
of the range has been cut down by the removal of the younger rocks, the 
core of the folded range shows itself in a variety of old granitic and 
gneissoid rocks, cut by intrusives and with extremely irregular structure 



26 GEEEN MOUNTAINS IN MASSACHUSETTS. 

We have done but little work towards the study of this old core. A 
valuable clew was found by Prof. Emerson in what he considers to be a 
threefold division of the pre-Cambrian iu southern Berkshire, where, 
according to his observations, chondroditic limestone separates a coarse, 
blue quartz gneiss — possibly the Stamford granitoid — from a still older 
gneiss. 

The massive granitoid gneiss which forms the core on Hoosac and 
Clarksburg mountains is in places separated from the overlying Cambrian 
quartzite gneiss series by beds of coarse, light-colored gneisses, which have 
interbedded layers of finer grain and darker color from the greater propor- 
tion of biotite. These "transitional coarse gneisses" between the granitoid 
and white gneisses are probably, to a great extent at least, Cambrian. 
They are detrital, containing pebbles in places, as in the tunnel. Their 
coarse feldspar is identical with that of the granitoid gneiss, except that in 
this transitional zone it is white, while in the granitoid it is reddish. While 
the granitoid gneiss is preeminently a massive rock, this "transitional" zone 
is bedded and contains micaceous layers. On the east side of the granitoid 
area on the surface of Hoosac mountain it occupies the place of the quartz- 
ite-white-gneiss-conglomerate and is overlaid conformably (as seen at the 
contact) by the albite-schists. The granitoid gneiss was probably much 
disintegrated at the time of the Cambrian transgression, and in the differ- 
ent conditions of character of disintegrated material and of breaching and 
sedimentation lies, perhaps, to a considerable extent, the explanation of 
the fact that this horizon is here quartzite and there gneiss, and presents 
itself under a great variety of aspects, due to alternating layers with vary- 
ing proportions of quartz, feldspar, and mica. But in the field it is often 
very difficult to distinguish, in the absence of true pebbles and of alter- 
nating sediments, between the redeposited detritus of disintegration, which 
has been subjected to the action of chemical and dynamic metamorphism, 
on the one hand, and beds which, simulating these, have been produced 
by the action of these same metamorphic agencies directly upon the older 
gneisses, granites, or basic eruptives. 

I imagine that the Cambrian transgression found an Archean elevation 
forming the western border of an Archean dry region. To the west of this 



GENERAL STKUCTURE AND CORllELATION. 27 

lay the great Paleozoic ocean of America. I imagine, also, that the rocks 
of this dry area had become disintegrated to a greater or less depth and 
that the products of this action varied from kaolin and quartz at the surface 
to semikaolinized material with fresh cores at depths. The depth of this 
action would vary according to varying lithologic and topographic condi- 
tions, as I have shown elsewhere.' 

While the abrasion of the deeply disintegrated rock was progressing 
along the advancing beach line the detritus of sand and pebbles arising 
from this disintegrated material was deposited with varying proportions of 
its constituents in a continuous sheet in progressive "transgression" over 
the previously -dry land;^ for I think the evidence offered by the erosion of 
the Stamford dike is sufficient to show that the region owed its absence of 
older sediments to its having been an area of dry land instead of an "abyssal" 
area. 

During the progress of this removal and deposition of ready-prepared 
material there would be places where the underlying unaltered rock would 
be washed clean and re-covered with sand and gravel. There would be 
others where the material removed from the disintegrated mass would be 
derived from the zone of semikaolinized fragmentary disintegration, and 
places where this material would be deposited without having been much 
rolled and in beds alternating with finer material. And again there would 
be places where the disintegration was deeper — in basins as it were — and 
where this material escaped removal and was covered by the sedimentary 
beds. The recognition of these premises would, it seems to me, aid in the 
explanation of many of the difficult points observed in the field. 

Take, for instance, the schistose lamination of the Stamford gneiss on 
Clarksburg mountain, where this structure is most highly marked near the 
contact with the overlying quartzite. The lamination is parallel in both rocks. 
The quartzite here bends around the mountain and is highly crinkled, this 
structure being defined by the micaceous constituent, and for some distance 

' Secular rock disintegration, etc. Am. Jour. Sci., vol. 17, 1879, pp. 133-144. Also the applica- 
tion and extension of the ideas advanced in that paper. F. von Richthofen: China, vol. 2, p. 758. 

'^ F. von Richthofen has called attention to the fact that toe little importance has been attached 
by geologists as a rule to the breaching and abrading action of the ocean when the beach line is 
advancing landward. China, vol. 2, p. 768. 



28 GEEEN MOUNTAINS IN MASSACHUSETTS. 

inward the same structm-e is similarly defined in the granitoid gneiss and 
is perfectly conformable in the two rocks, although we have here, in the 
conglomeratic character of the base of the quartzite and in the pre-Cambrian 
erosion of the Stamford dike, evidence of a time-break. If we imagine the 
granitoid gneiss to have been deeply disintegrated and to have been abraded 
only to the semidisintegrated zone, or even to the lower zone in which 
only the integrity of the micaceous element had been attacked, then the 
material of this zone would have presented itself to the force that produced 
the crinkling and lamination in much the same physical condition as the 
sand and pebbles of the quai-tzite. 

Again, take the coarse gneisses with blue quartz which occur at many 
points along the core. Mr. Wolff finds them to contain the same feldspar 
with the same inclusions as that of the granitoid gneiss, except that they are 
light colored, while those of the granitoid are reddisli, and thev have fre- 
quently the same blue quartz. But they are bedded and have alternating 
layers of finer schists, and a2)2jear as transitions conformable to the under- 
lying granitoid and overlying white gneiss or other equivalents of the Cam- 
brian quartzite. The granitoid gneiss consists of large crystals of feldspar — 
perhaps averaging one by three-quarters by one-third inch in size — and 
flattened lenses of blue quartz and thin, irregular layers of mica. I imagine 
that these materials, taken from the zone of semidisintegration and quickly 
deposited, would, in their new arrangement, produce our "transitional 
coarse gneisses," while the material of the upper zone of complete decay 
would furnish the sand and clay for the quartzite and finer sediments. 

If this reasoning be correct, we should in many instances include in 
the Cambrian quartzite series the coarse, more thinly bedded gneisses, with 
then- iuterbedded, finer grained schists. But in the present state of our 
knowledge of the Grreen mountains the granitoid gneiss appears to be only 
one of the constituents of the old core, and perhaps a subordinate one. 
From our recent work in Vermont it seems that the pre-Cambrian area will 
be found to contain a variety of granites, gneisses, and schists, as well as 
basic rocks, which will need to be studied in connection with the rocks of 
both the New York highlands and the Adirondacks. It therefore remains 
to be discovered whether the old core contains any rocks of the periods be- 



GENERAL STRUCTURE AND CORRELATION. 29 

tween the Archean (Laurentiau) and Cambrian. Thus fai* only some ob- 
servations that will serve as clews have been made in this direction/ One 
apparently negative piece of evidence may be seen at the place where the 
Archean rocks of the New York highlands suddenly end near Poughquag, 
Dutchess county, New York. Here the highlands end in a promontory 
of nearly vertical beds of old gneisses, against which the Cambrian qtiartz- 
ite lies with a very flat dip. 

Toward the correlation of the Green mountain rocks with the fossilif- 
erous strata of New York, the paleontologists have given us some facts. 
Mr., Walcott's discovery of Olenellus casts in the quartzite of Clarksburg 
mountain, about 100 feet above its base, caused him to assign that rock to 
the Lower Cambrian. The many findings of Lower Silurian fossils in the 
limestone of Vermont have shown that limestone to include Calciferous, 
Chazy, and Trenton horizons, and it is inferred that, since the limestone is 
Trenton and is capped by schists, the latter are of the age of the Utica and 
Hudson River slates. 

I have shown above that the white gneisses and conglomerates of 
Hoosac mountain are the equivalents of the Cambrian quai'tzite and that 
the albitic schists of Hoosac mountain represent in time both the limestone 
and schists of the valley, and therefore range from the Cambrian into or 
through the Hudson River. 

It seems probable that the limestone must reach down well into the 
Cambrian and that all of the Cambrian that is not represented by the 
quartzite must, in the valley, be included in the lower part of the limestone 
and its downward transition beds;^ while on the mountain it must be in- 
cluded in the lower beds of the albite schists. 

We have yet to discover whether the nonfeldspathic schist of the 
eastern portal of the tunnel (Rowe schist) represents Hudson River, or, 
Ijerhaps, Medina time. Geologically above the nonfeldspathic schists of 
the eastern portal, and coming in successively to the east to build up the 
old plateau region that forms properly the eastern belt of the Green moun- 

' Since this was written we have fonnd Algonkiau schists at several points along tlie Green moun- 
tains. 

- This has been confirmed by recent discoveries of Cambrian fossils in the lower part of tlie lime- 
stone near Rutland and Clarendon, Vermont, by Messrs. Foerste, Wolft', and Dale. 



30 GREEN MOUNTAINS IN MASSACHUSETTS. 

tain system as far as the Connecticut valley, there is a series of schists 
having a great aggregate thickness. Prof Emerson, to whose report the 
reader is referred for the descriptions and for the views of our predecessors, 
has been able to divide these schists into several distinct formations with 
persistently defined characters and boundaries. Above the nonfeldspathic 
Rowe schists comes a horizon of hornblende schist (Chester amphibolite) 
often with serpentine, varying from a feather edge to 3,000 feet in thick- 
ness, overlain by over 9,000 feet of "upper hydromica-schist" (Plainfield 
schist). This in turn is overlain by the "Calciferous mica-schist" of the 
Vermont survey (Conway schist), which obtained its former name from the 
presence of occasional large lenses of more or less biotitic limestone, which 
latter has beds of hornblende-feldspar-schist in places along its bottom and 
top. Above this again is the heavy bed of Leyden argillite, with inter- 
calated quartz-schist. Next above and unconformably supeq^osed are the 
representatives of the Devonian. The age of these different fonnations 
still remains uncertain, though at least the Leyden argillite and the Conway 
schist ("Calciferous mica-schist") are supposed by Prof Emerson to belong 
to the Upper Silurian. 

While the Green mountain system includes the whole region between 
the Connecticut and the Hudson, its characteristic features consist, as we 
have seen, of the central anticlinal ridge of the Green mountains proper 
on the east, the synclinal range of the Taconic mountains on the west, and 
a succession of high, synclinal, island-like masses rising from the intermedi- 
ate valley. The results of the survey in northwestern Massachusetts lead 
to the supposition that the central or main ridge was in pre-Cambrian time 
outlined as a mountain range of highly crystalline rocks on the western 
border of an area of dry land. During long exposure to the action of 
atmospheric agencies and of the products of vegetable decay, the rocks of 
this region had become decomposed at the surface and disintegrated at 
depths. 

The breaching action along the advancing shore line of the Cambrian sea 
found ready prepared the materials which the water assorted and distributed 
to form the great sheet of Cambrian rocks. While these deposits of detritus 
were accumulating over the shallow areas, the mateiials for the future lime- 
stone were gathering offshore to the west. As the positive movement 



GENERAL STRUOTUEE AND CORRELATION. 



3] 



deepened the water shoreward, the calcareous materials accumulated above 
the earlier detrital beds, so that we may imagine that, while the later 
beds of the Cambrian were being made of sand and gravel in shallow 
water, the lower beds of the great limestone formation were being deposited 
offshore. Later, with a change of some kind in the conditions, there came 
the deposit of finer material over the previously shallow region, while the 
accumulation of limestone, with Lower Silurian organisms, still continued 
offshore. Still later, by another change in the conditions, the deposit of 
finer detrital material extended far to seaward, covering everywhere the 
limestone accumulations. 







u 



Stanibrrl TransvUonat 
Gneiss Gn^s s 







FiQ. 8.— Map showing the varying character of the Cambrian rocks in con- 
tact with the pre-Cambrian granitoid gneiss mass on Hoosac mountain. 

As we are not yet able to say to what depth into the Cambrian the lime- 
stone may extend in the Hoosac valley, so, also, we are unable to say to 
what extent the lower beds of schists on Hoosac mountain may represent 
Cambrian time. 

Mr. Wolff has shown that the Cambrian quartzite horizon, which is a 
true conglomerate on the top of the arch at the north end of the granitoid 
gneiss area, consists on the eastern and easterly dipping limb of coarse 
gneisses, showing only occasional pebbles, as in the tunnel, while on the 
western and crumpled limb it is represented by finer-grained white gneiss. 
These relations are shown in Fig. 8. We may suppose an island of 
coarse granitoid gneiss with a disintegrated mantle, and imagine this latter 
to have been abraded down to its less disintegrated zone, and the resulting 



32 GREEN MOUNTAINS IN MASSACHUSETTS. 

coarser material to have beeu laid down, during the positive movement, over 
the gneiss area. In the subsequent folding I imagine that the rigidity of 
the unaltered granitoid mass offered far greater resistance to the folding 
than any of the superposed material, and that, as a result of this resisting, 
inverted wedge, the material of the eastern limb was subjected to the slip- 
ping or shearing movement producing the coarse laminated structure of 
these gneissoid rocks, while the similar material on the west limb, having 
a more rigid base which yielded less readilv to overfolding, was forced into 
minor overfolded crumples and crushed into a finer grain. Beneath the 
gneisses remade out of the conglomerate by dynamic action during the 
folding, there would be formed more or less similar transitional rocks 
through the action of the same dynamic processes upon the semidisinte- 
grated surface of the older rock. This is what is found at many points 
along this contact in Hoosac mountain. 

From what has just been said it is evident that the high degree of 
metamorphism of the Paleozoic rocks is intimately connected with the 
folding. It is also a salient fact that, while the schists and limestone are 
wholly recrystallized throughout the whole folded area beginning west of 
the Taconic range, the change of the underlying Cambrian quartzite to 
a crystalline rock — a white gneiss — does not begin until, in going east, Ave 
reach the central, main range. In this sense the metamorphism of the 
schists is regional ; that of the quartzite has the apjDearance of being local. 

Both the quartzite and the ovei'lying schists contain tourmaline, crys- 
tallized in situ, and frequent lenses or faulted veins of quartz, feldspar, and 
tourmaline. The schists contain in places needles of rutile. As we follow 
the quartzite in its transition to white gneiss we find here and there peg- 
matite veins, more often near its contact with the core of older gneiss. 

If we could go back to the original character of the sediments we would 
find west of the western flank of Hoosac mountain a column of fine sedi- 
ments, probably argillaceous, with, in places, calcareous bands, resting on a 
thousand feet or more of limestone, and this on six or eight hundred feet 
of Lower Cambrian grit — here a quartz sandstone. On the eastern side of 
the western flank of Hoosac mountain we would find many thousand feet 
of the same fine sediments resting on, and passing downward into the 
Cambrian grit — here a coarse conglomerate abounding- in detrital feldspar 



GENERAL STRUCTURE AND CORRELATION. 33 

in its cement. We would find the limestone of the western column repre- 
sented only by more or less calcareous matej'ial in the fine sediments of 
the corresponding part of the eastern column, and by a rather abrupt 
lateral transition through flaggy limestones and marls, containing more 
quartz sand at the bottom and more clay at the top. Above this horizon 
we would find the fine sediments alike common to both columns and 
extending far both to the east and the west. 

Analyzing the different horizons we find along the west side of Hoosac 
mountain different conditions of sedimentation aff'ecting the horizons of both 
the grit and the limestone. To the east the grit becomes a conglomerate 
abounding in granitic pebbles and in detrital feldspar. To the east also the 
limestone passes into shoreward argillaceous sediments. Higher up we find 
in the uniformly widespread fine sediments the evidence of changed condi- 
tions, which through a long period excluded to a great extent the formation 
of limestones over the whole region. 

Such in a general way was the differentiated character of the rocks upon 
which the processes of metamorphism acted. These processes resulted in 
changing the quartz sandstone of the Cambrian grit into a quartzite, and the 
shoreward feldspathic sandstone into a highly crystalline gneissi. The Cam- 
bro-Silurian limestone, the limestone proper, was changed to crystalline 
limestone; its shoreward transitions into more or less calcareous gneiss and 
its more eastward calcareous shales into a garnetiferous variet}^ of the albitic 
schist, into which the whole column of Cambro-Silurian fine sediments above 
the lower Cambrian grit has been changed. In the finer sediments, the 
uniform character above the horizon of the limestone resulted in a uniform 
change into a mica-schist characterized by the general presence of albite in 
macroscopic or microscopic crystals. 

We do not yet know to what depth these rocks were buried. They 
ha.ve in themselves an aggregate thickness of 5,000 feet or more. Certainly 
if they were covered by the great thickness of material represented in the 
schists between Hoosac mountain and the Connecticut river, they were 
buried to a point of load and temperature sufficient to satisfy these condi- 
tions of metamorphism. 

Throughout the whole region all the rocks above the pi-e-Cambrian 

MON XXIII 3 



34 GREEN MOUNTAINS IN MASSACHUSETTS. 

have been subjected to the action of great lateral pressure, throwing them 
into folds and along certain lines into compressed and ruptured overfolds, 
subjecting the constituent particles to crushing or shearing and to move- 
ments which are now marked by the crinkling of the original stratification 
lamination, and by the predominant cleavage resulting from movement. 

There were therefore present the three factors of load, temperature, 
and attrition of particle on particle produced during the folding movement. 
These factors were essential in the process of metamorphism, but they could 
not change ordinary clay sediments into schists consisting largely of mag- 
nesia and potash micas and abounding in soda-feldspars, nor could they 
change a grit of quartz and microcline detritus into a gneiss consisting largely 
of soda-feldspar. Either the original sediments must have contained all of 
the elements required to form by recrystallization the present constituent 
minerals, or a part must have been contributed from elsewhere. The 
extreme rarity of observed eruptive dikes and of pegmatite veins outside 
of limited areas makes it hard to explain the difference between the chemical 
constitution of the schists in their great breadth and thickness and that of 
ordmary argillaceous sediments by ascension from below. It would there- 
fore appear more likely that the original sediments were of an exceptional 
character. They may have been deposited under conditions favorable to 
the preservation of magnesium and alkaline salts — conditions which we know 
have at ^various times existed over large areas. 

In the case of the Lower Cambrian grit the action of mineralizing 
processes originating below is more pi-obable. Where the rocks have been 
subjected to the different forms of readjustment of particles during the great 
folding of the strata, a change occurs from a gi'it containing much detrital 
microcline to a highly crystalline gneiss with a predominant soda feldspar, 
Avhich bears evidence of being crystallized in situ. Along these zones 
we find veins and "flames" of pegmatite, and in the crushed quartzite proper 
perfect little crystals of tourmaline often appear in great abundance. The 
very feldspathic veins along these zones of extreme folding in the grit may 
stand related causally to the lenses of qiiartz and tourmaline, with and 
without feldspar, which occur rather frequently in the higher schists along 
the west flank of Hoosac mountain; also along the zone of extreme folding. 



\ 



F^RT II. 



THE GEOLOGY OF HOOSAC MOUNTAIN 



ADJACENT TERRITORY. 



J. E. M^OLFF. 



35 



CONTENTS 



Page. 

Introduction 41 

Topograpliic! work 41 

Toijography 41 

Description of rocks of Hoosac mountain 44 

The Stamford gneiss 45 

The Vermont formation : 48 

The Hoosac schist 59 

> The Stockbridge limestone 64 

Amphibolites 65 

Geology 69 

The Hoosac tunnel 69 

The region embracing the central part of Hoosac mountain 72 

The northern and eastern schist area 86 

The region south of Cheshire and of tlie Hoosic valley 88 

Hoosic valley schist 97 

The region around Clarksburg mountain and Stamford, Vermont 98 

General conclusions 102 

Descriptions of plates 109 

37 



ILLUSTRATIONS 



XI. ^ 



Page. 

Plate IV. Detailed map of western crest and slope, Hoosac mouutain 40 

V. Geologic proflle.s, Hoosac mountain 70 

VI. Geologic profiles, generalized, Hoosac mountain 80 

VII. Thin sections, white gneiss 110 

VIII. Thin sections, white gneiss and albite schist 112 

IX. Thin sections, diorite and amphibolite 114 

X. Thin sections, quartzite conglomerate and crumpled metaraorphic conglomerate 116 

( View north over crest of Hoosac mountain 118 

B \ Profile of Hoosac mountain from Spruce hill south, looking west 118 

Fig. 9. View from Hoosac mountain _. 43 

10. Profile of Hoosac mountain (western crest) 43 

11. Profile of Hoosac mountain (western slope) 44 

12. Granitoid gneiss 45 

13. Metamorphic conglomerate, showing crushing 48 

14. Metamorphic conglomerate, showing shape of pebbles 49 

15. Metamorphic conglomerate, flattened pebbles 50 

16. Metamorphic conglomerate, round and flat pebbles 51 

17. Metamorphic conglomerate, banded variety 53 

18. Metamorphic conglomerate, typical 55 

19. Metamorphic conglomerate, showing large pebbles 57 

20. Conglomerate, clitf 58 

21. Albite-schist, Hoosac schist 59 

22. All)ite-schist, Hoosac schist... 61 

23. Albite-sohist, Hoosac schist 62 

24. Mount Holly amphibolite 65 

25. Mount Holly amphibolite 66 

26. Mount Holly crumpled amphibolite '. 67 

27. Contact of granitoid gneiss and metamorphic conglomerate 73 

28. Contact of granitoid gneiss and quartzite, Stamford dike, looking north 100 

29. Contact of granitoid gneiss and quartzite, Stamford dike, looking east 101 

39 



3 GEOLOGICAL SURVEY 



MONOGRAPH XXin PL T/ 




THE GEOLOGY OF HOOSAC MOUNTAIN AND ADJACENT TERRITORY. 



By J. E. Wolff. 



INTRODUCTION. 

The territory embraced in this report extends from the Hoosic valley 
in the west to the meridian of 73° on the east, and from the state line on 
the north to the valley in the south which runs east from Pittsfield through 
Dalton. It covers the easterly half of the " Greylock sheet" of the new map 
of Massachusetts. It is an area about 18 miles in length, varying from 10 
to 4 miles in width, and covering about 120 square miles. 

TOPOGRAPHIC WORK. 

As the extreme complication of the field required great accuracy in the 
location of ovitcrops, at an early stage in the work a base-line 7,000 feet 
long was meastired on the Boston and Albany railroad in Hoosic valley and 
a sufficient number of points were established by triangulation to allow the 
accurate vertical and horizontal topographic determination of important out- 
crops, which were then plotted on a large field map on a scale of 1,000 
feet to the inch. Subsequently the plane-table sheets of the state map (scale 
2 inches to the mile) were utilized, and a special topographic map of that 
part of Hoosac mountain near the tunnel (on a scale of 1,000 feet to the 
inch) was prepared. At many places accurate section lines were run by the 
stadia and the geological points incorporated in the general map. 

TOPOGRAPHY. 

Hoosac mountain is the name applied to a part of the Green mountains 
situated in the northwest corner of the state of Massachusetts, near the Ver- 
mont boundary. This region forms the watershed between the Hoosic and 

41 



42 



HOOSAC MOUNTAIISr. 



Deei-field rivers, branches of the Hudson and Connecticut, respectively. At 
its southern end it is di-ained by branches of the Hoosatonic and by the 
Westfield river, a branch of the Connecticut. 

The entire mountain mass is cut through at its central part from east to 
west by the Hoosac tunnel, nearly 5 miles long, the tunnel passing almost 
directly under the highest point of this part of the mountain, a knob one- 
half mile north of Spruce hill, which is 2,600 feet above the sea. At the 
extreme north of the field, half a mile south of the Vermont line, the highest 
point is found to be 2,800 feet. Where the tunnel crosses the central part 
of the mountain the outline is that of a double crest with a central basin or 




Fig. 9.— View looking west from alope of Hoosac mountain, east of North Adams. This gives a general idea of the 
topography of the valley. 

depression (see Profile iii, PI. v), the two sides joining at the north end to 
form the high north point and to terminate the basin. 

In its southern-central portion the mountain loses the north to south 
ridges and drainage. It is tliere characterized by flat, rounded summits 
and gentle depressions, and a frequent east to west trend of the valleys. 
A glance at the strike and distribution of the formations will show that the 
frequent east to west strike and extreme crumpling of the white gneisses 
which occupy this region cause this diff"erence in the topography. In 
the southern part of the field a north to south strike of considerable regu- 



GREEN MOUNTAmS IN MASSACHUSETTS. 



43 



larity again comes in, causing a more ridge-like topography, until the deep 
east to west valley of Dalton, a mile or so south of the map (PI. i), bounds 
the region on the south. 

On the east the mountain joins the hilly country extending to the 
Connecticut river; on the west the bi'oad Hoosic valley, running noilh and 
south, bounds Hoosic mountain and separates it from the mass of |Grreylock 
mountain, the highest in the state. (See Fig. 9). ( 

The relations of topography to geological stnicture are ofbfen notice- 
able. The whole eastern border of the area shown on the map is covered 




Fig. 10 — Profile of part of west crest of Hoosac mountain, looking cast from Hoosic valley opposite Adams. 

This shows the continneil northerly pitch of the axissome miles sonth of point sho\Yii in PI. XI, B. The snmmit in the 
right center is of white gneiss (Vermont formation) with a little indistinct minor ridge of the Hoosac schist trough, both 
slanting to the left (north). 

by the schists, characterized by a uniform north to south strike and steep 
easterly dip of their structural planes; and the ridge topography, with deep 
cross-goi'ges for the streams, is evidently due to that structure. 

The long crest of Hoosac mountain, forming the main watershed, 
coincides in direction and position with the axis of the northerly pitching 
fold which forms the principal feature of the mountain, and with the axis 
of the central core of granitoid gneiss. The presence of the limestone in 



44 



GEEEN MOUNTAINS IN MASSACHUSETTS. 



the Hoosic and Dalton valleys determined these depressions, as is always 
the case with that rock. 

The profile of Hoosac mountain shows plainly the northerly " pitch" ^ 
of tlie formations by the gentle slopes to thQ north and the bluffs facing 
south. (See Fig. 10 and PI. xi, b.) The western slopes of Hoosac mountain 
running down to the Hoosic valley are steep, but have a marked series of 
buttresses or benches. (See Fig. 11.) The ckift-covered Hoosic valley is 




Fig. 11. — Profile of west slope of Hoosac mountaiD, from Hoosic valley opposite Adams, looking north. 
This figure shows the buttressed character of the west slope of the mountaiQ at the left ceuter. These IfuttreBSes are 
of crumpled white gneiss (Vermont formation), with a gentle easterly dip. 

comparatively flat, sending branches into the mountain, Avhich are locally 
called "coves." At Cheshire the valley makes a sharp turn to the west. 

DESCRIPTION OF THE ROCKS OF HOOSAC MOUNTAIN. 

The rocks of this region are thoroughly crystalline, but little trace 
remaining in general of their original elements, whether of detrital or erup- 
tive origin, but the bedding corresponding to the original planes of deposit 
is well marked, and, under the proper conditions, we can therefore deter- 
mine the order of succession. 



' Meaning that the axes of the folds are inclined or " plunge " in that direction. 



HOOSAC MOUNTAIN. 



45 



THE STAMFORD GNEISS. 

The basement rock is a coarse granitoid gneiss, which forms the core 
of Hoosac mountain proper, occupying the surface of the mountain for 
several miles, .then disappearing below the overlying rock, but cut in 
Hoosac tunnel for nearly 5,000 feet; hence this rock figures prominently 
on the dumps of the tunnel shafts. Another area of the same rock under- 
lies the fossiliferous Cambrian quartzite of Clarksburg mountain, north of 
Williamstown, continuing some miles northward into Vermont — the "Stam- 
ford granite" of the Vermont geological report. 




Fig. 12. — Granitoid gnei8.s (Stamford gneiss), from dump Central shaft. Natural size. 

Tliis is the variety with a well-marlied gneissoid structure. The dark streaks are composed of the micas inclosing 
irregularly lenticular areas of feldspar and quartz. 

In its most typical form the rock is a coarse banded gneiss (see Fig. 12), 
composed of long lenticular crystals of pinkish feldspar, flattened lenses of 
blue quartz, and thin, irregular, greenish layers of a micaceous element 
(biotite or muscovite, or both) mixed with small epidote crystals, which 
cause in part the greenisli color. We notice at once that tlie broad cleav- 
ages of the feldspar often do not reflect as one surface, but as a num- 
ber of little disconnected areas, which are often curved — a well-known 



46 GREEN MOUNTAINS IN MASSACHUSETTS. 

effect of great pressure in crystalline rocks. The feldspars contain little 
dull grains of quartz, black specks of mica, and crystals of magnetite, and 
are often crossed by little branches from the layers of mica outside. At 
the edges the feldspars often pass very irregularly into the quartz, which 
then forms the narrow parts of the lens of which the feldspai* forms the 
center (" Augen" structure). 

The quartz is characteristically blue, but when crushed by pressure in 
the rock is often white or sugary in appearance, the blue cores then rep- 
resenting the uncrushed material. In other varieties of this rock it has 
almost the structure of a coarse granite. The quartz is deep blue, the 
feldspar colorless and in Carlsbad twins, and the mica layers black. The 
gneissic structure is almost wanting. 

Certain other variations occur in the structure of the gneiss. In the bed 
of Roaring brook, Stamford, Vermont, the gneiss on the weathered surface 
has numerous rounded elliptical masses which by the absence of quartz 
and scarcity of mica stand out by contrast with the rock as a whole, and 
look like pebbles. They are composed of feldspar aggregates and flakes 
and patches of biotite. The microscope shows that these feldspars are mi- 
crocline with some plagioclase and perhaps orthoclase; they have the gen- 
eral structure of the gneiss itself, without the quartz, and are probably of 
contemporaneous origin. West of Stamford village the rock contains Carls- 
bad twins of microcline an inch or two across, which weather out from the 
rock, become rounded by decay, and look like pebbles. 

The microscopic characters of this rock are quite uniform; the large 
feldspars are generallv microcline,^ with whatever crystalline boundary they 
may have once possessed obliterated by the great mechanical changes they 
have undergone. The crystals are often faulted and the edges crushed; 
little veins of secondary quartz, mixed with little grains or crystals of an 
un striated feldspar (albite?) traverse them along the fault lines. (See PI. 
VII, B.) With a low power the feldspar substance appears cloudy, owing to 
fluid cavities ajid little prisms of epidote in great numbers. These epidote 
grains are sometimes arranged parallel to the twinning planes of the feld- 



' In the " Geology of Vermont," vol. 2, p. 561, there is an analysis of the feldspar from the Stam- 
ford granite, according to which it contains from 64 to 66* per cent silica, 10 to 11 per cent potash, and 
2 to 3i per cent soda. 



HOOSAC MOUNTAIN. 47 

spar, sometimes not. In some localities the feldspar contains little round 
red garnets. Flakes of biotite and muscovite and octaliedra of magnetite 
are common inclusions. 

The quartz masses show cataclastic changes in the same way; the 
original cores of the blue quartz, themselves somewhat strained (seen by 
using polarized light), are surrounded by masses of broken quartz, the 
derivation of which from the parent mass can easily be traced. The finer 
grained portions of the rock are composed of little fragments of microcline 
broken off from the larger pieces, and small simple crystals, often simple 
twins, of a feldspar which shows but rarely the multiple twinning of pla- 
gioclase, and which resembles the albite of the schists. The layers of mica 
are composed of muscovite, often with a greenish color like talc, but easily 
identified by the large axial angle, flakes of dark brown biotite, rarely 
altered to chlorite, crystals of magnetite, and the omnipresent epidote in 
prisms or small grains mixed with the micas or inclosed in them. There 
are occasional imperfect crystals of apatite and prisms of zircon. Some of 
the magnetite grains are titaniferous, as can be seen by the yellow border of 
titanite derived from them. In many slides there are quite large crystals of 
feldspar which have no multiple twinning, extinguish parallel to the cleav- 
age, and are perhaps orthoclase. 

Slides of the large porphyritic Carlsbad twins show that they are micro- 
cline, filled with irregular bands of a feldspar which extinguishes parallel 
to QoP do, and is filled with epidote crystals. Aggregates of biotite plates 
associated with hornblende crystals are common. There are also masses of 
ilmenite altered in part to titanite. Sometimes circles of hornblende crys- 
tals and biotite plates, which inclose a core of aggregate quartz, by their 
shape and occurrence suggest a possible replacement of garnets. Grains of 
quartz and crystals of zircon are common, so that nearly all the constitu- 
ents of the rock occur within these cr)rstals. 

What may have been the origin of this rock it is impossible to say with 
certainty; it is evident that crushing and the development of mica, quartz, 
and feldspar, parallel to planes of break and sliding has had a great deal to 
do With the development of the parallel structure. Viewed from this 
standpoint it could perfectly well have been an eruptive granite modified 



48 



GEEEN MOUNTAmS IN MASSACHUSETTS. 



by metamorphism. On the other hand, its field relations show its close asso- 
ciation with and frequent transition into coarse gneisses which seem to form 
part of a detrital series. 

THE VERMONT FOEMATION. 

A somewhat varied series of rock overlies this coarse basement gneiss. 
At one place where there is no possibility of folding (namely, along the 
pitching axis of Hoosac moimtain (see PL v, Profiles ix, x). The thickness 
of this series has been measured between a conformable contact with the 
granitoid gneiss below and one with the albite-schist above; it is between 
600 and 700 feet. 




Fig. 13. — Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-fiftli natural size. 

This represents two faces of one block at right angles to each other, the line showing the corner. The pehhles are of 
granulite and blue quartz, some of them 1^ inches in diameter. The ditferent shape of tlie cross-sections in the two planes 
is noticeable. By looking closely it will he seen that many pebbles are cut in two hy dark lines (biotite), showing that 
their present shape is due partly to crushing and the formation of new minerals. This is seen on the right side, but not on 
the left. 

This formation contains an infinite series of gradations between coarse 
gneisses similar to the basement gneisses, finer grained banded gneisses, 
gneisses composed of quartz and feldspar with but a small amount of the 
micaceous element, metamorphic gneiss-conglomerates, ordinary quartzite- 
conglomerates, and quartzites. This series of rocks (represented by gneisses 
and metamorphic conglomerate) occupies a position in the tunnel section 
on either side of the central core of granitoid (Stamford) gneiss; while a 



flOOSAC MOUNTAIN. 



49 



second nairow belt occurs near the West Portal, adjoining the Hoosic 
valley (Stockbridge) limestone. On the surface it occupies a large area, 
especially in the southern part of the field. The quartzites occur generally 
in or near the Hoosic valley adjacent to the limestone, associated with 
conglomerates and passing along the strike into the granulitic and gneissic 
rocks by increase in the amount of feldspar and mica. . 




Fig. 14.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. Two-ninths natural size. 
I'lie larger pebbles are mostly granulile, with some of blue quartz and feldspar. Tliis shows very plainly the shape of 
the pebbles, which are but little elongated in the plane normal to the picture. 

Beginning with the simplest rocks, the quartzites, there are vitreous 
varieties and crumbly feldspathic varieties passing into gneiss. The 
vitreous variety occurs in large masses with very obscure stratification, 
and roughly jointed. It varies in color from snow white to yellow, contains 
often layers of mica and cubes of pyrite. The microscope shows an even 

MON XXIII 4: 



50 



GEEEN MOUNTAINS IN MASSACHUSETTS. 



grained, closely interlocking aggregate of little rounded or irregular quartz 
grains mixed with considerable feldspar in similar in-egular grains. Broken 
or rounded crystals of apatite and zircon and perfect crystals of tourmaline 
and rutile are common. The feldspar grains are in part microcline, 
plagioclase, and an untwinned feldspar (orthoclase ?). Unless it be the 
apatite and zircon, no unmodified original clastic elements can be recognized 
in this rock. According to the usual view of the origin of quartzite, the 
quartz grains have been enlarged by the growth of new silica, so that the 
original form is wanting, and the feldspar, judging from its similarity to 
that of other i-ocks in which it is undoubtedly metamorphic, has probably a 
similar origin. 

In many localities the quartzites have a crumbly character, so that 




Fig. 15.— Metamorphic conglomerate (Vermont formation), near contact with granitoid gneiss. Top of Hoosac moun- 
tain. Fallen block. One-twentieth natural sixe. 

This also pbows the production of flattened " pebbles " by crushing and the development of biotite, etc., along crushing 
and slipping planes. In the right hand of the picture this is especially clear. The pebbles here are granulite, passing 
into a line grained granite. 

they can be picked or shoveled out, and are extensively quarried for glass 
sand. Prof J. D. Dana has called attention to this^ and suggested weather- 
ing as a cause, and connected it with the alteration and leaching out of the 
feldspar. In some of the quarries the percolating water cames down filie 
kaolin, and forms beds of pipe clay in the bottom of the quarry. But some 

' On the decay of quartzite, and the formation of sand, kaolin, and crystallized quartz. Am. 
Jour. Sci., 3d ser., vol. 28, 1884, p. 448. 



HOOSAC MOUNTAIN. 



51 



of these crumbly qiiartzites show but little feldspar and that quite fresh, 
while the quartz grains show in the slide abundant signs of great pressure, 
or even crushing. Some of these quarries are located at sharp folds of the 
quartzite, so that the crumbly nature of the quartzite may be in part due 
'' to a mechanical loosening of the cohesion. 

The pure conglomerate-quartzites occur often in the quartzite; for 
instance, the quartzite resting on the granitoid gneiss (Stamford gneiss) of 
Clarksburg mountain, in which Mr. Walcott has found fragments of trilo- 
bites.^ contains pebbles of blue quartz, which are often only distinguishable 




Fig. 16. — Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-sixth natural aizo. 

The pehbles are mostly granulitic, but there are some of blue, and some of white quartz. In this type we have round 
and fiat pebbles ocourring together, the round ones differing but little in shape in the normal plane. This represents 
the typical variety of conglomerate. 

by their color from the surrounding quartzite cement. The microscope 
shows that many of these pebbles are composed of an aggregate of little 
quartz grains derived from a homogeneous mass by crushing, and hence 
they easily blend with the quartz cement of the rock. They occur often 
in flattened elongated forms which it is difficult to distinguish from secre- 
tions. (See PI. X, B.) 

Some of the quartzites contain abundant calcite grains arranged in 
stringers, and scattering flakes of muscovite. 

Those quartzites in which feldspar becomes more prominent preserve 

' Am. Jour. Sci., 3d ser., vol. 35, 1888, p. 236. 



52 GEEEN MOUNTAINS IN MASSACHUSETTS. 

still the appearance of the purely quartzose varieties. The feldspar occurs 
in irregular grains fitting in between the quartz. It is partly not twinned 
(orthoclase?), part plagioclase, generally microline. Little crystals of rutile, 
prisms of zircon and apatite, flakes of biotite and muscovite, masses of iron 
hydrate, pyrite, etc., occur in nearly all the specimens. The quartz and 
feldspars often show evidence of mechanical crushing, and part of the quartz 
has been thus derived from larger grains. The constituents have a nearly 
even grain. Although garnet is very rare, it is convenient to call this rock 
the granulitic type of the quartzite. 

From this rock tlie transition is easy to the white gneiss proper. Sev- 
eral varieties of this may be recognized; a banded one is common, the color 
varies from gray, yellow to white; sometimes the banding is very fine or 
the rock is speckled with biotite or muscovite, or both ; sometimes the feld- 
spar forms layers separated by layers of mica, or occurs in rounded or irreg- 
ular masses. The proportions of the elements vary in every conceivable 
way. 

A characteristic feature in the slide is seen in the round grains of feld- 
spar of someAvhat larger size than the average of the rock, inclosed in a 
groundmass composed of little grains of quartz and feldspar in a most 
intimate admixture, while plates of mica give the rock its banded struc- 
ture. The larger sized feldspars are typically of a peculiar rounded shape, 
occurring either in single crystals or in broad simple twins. They are com- 
monly entirely without the polysynthetic twinning of plagioclase in polar- 
ized light and might be taken for orthoclase, but their isolation from the 
powdered rock by the Thoulet solution shows by the specific gravity that 
they must be generally a soda-lime feldspar near the albite end of the series, 
although in some rocks they must cbntain considerable lime, judging by theii* 
specific gravity. These feldspars are commonly filled with inclusions of 
minerals found in the rock outside them — little prisms of epidote, flakes of 
biotite and muscovite, and little rounded grains of quartz, sometimes 
arranged like a necklace. These inclusions often lie in planes parallel to 
the arrangement of the minerals outside the feldspar, and entirely inde- 
pendent of crystallographie directions in the feldspar. (See PI. vii, a, 
and PI. VIII, A.) It is very rare to find any sign of mechanical deformation 
in these feldspars. 



HOOSAC MOUNTAIN. 



53 



Quartz occurs sometimes in large rounded masses, greatly strained and 
shattered, and surrounded by a mosaic of small quartz grains. Large pieces 
of microcline occur, faulted and broken, the cracks filled with an aggregate 
of little quartz grains and feldspars in simple twins. (See PI. vii, b.). The 
groundmass of the rock is a closely interlocking aggregate of quartz grains, 







k 



\ 



^ 



Fig. 17.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-fifth natural size. 
The tiy,nre represents the banded variety of the rock, in which vre find it difficult to draw the line hetween true pebbles 
and forms produced by crushing. A glance at the figure, especially at the right side, shows that the extremely pointed ends 
of some of the apparent pebbles must he produced by the encroachment of the mica layers. Yet these white masses have 
a lithological character different from that of the "cement," forming, for instance, the broad baud near the rightside. The 
former are a fine grained granite or granulite, sometimes blue quartz ; the latter a coarser grained mixture of quartz, mica, 
and some feldspar. 

little feldspar gi^ains simply twinned (if at all) and often little grains of 
microcline of the same form and size. Epidote is often present in large 
quantities, foiTning microscopic yellow bands in the rock, and inclosed in 
the feldspars and micas in little prisms and grains, but not in the quartz. 



54 GREEN MOUNTAINS IN MASSACHUSETTS. 

Titanite, rutile, aud tourmaline occur sparsely, as well as little broken prisms 
of apatite and zircon prisms. The micas occm* in homogeneous plates ; the 
interwoven sericitic structure is not common. Magnetite occiu's occasionally. 

Another variety of these gneisses is distinguished by the evenness of 
its character and its occurrence along the base of Hoosac mountain as the 
most western band of the gneisses, in close connection with the quartzites 
and limestones. The rocks thrown out from the "well" shaft, a few hun- 
dred feet west of the west shaft of the tunnel, are typical of this variety. 
In the hand specimen the rock is a fine grained, evenly banded gray gneiss; 
the minerals are arranged in layers and the rock is filled with little 
squarish feldspars. In the slide these feldspars are seen in gently rounded, 
eqiiidimensional crystals, in simple twins, according to the albite law. The 
groundmass is composed of little round or ellipsoidal quartz grains and 
more angular pieces of feldspar (which are in part in simple grains, in part 
doubly twinned microcline) mixed with threads of muscovite and biotite, 
the whole so arranged as to produce a schistose structure in the rock. (See 
PI. VII, A, and PI. VIII, a.) Sometimes a band of mica and quartz cuts across a 
feldspar, the two halves polarizing together and being therefore part of one 
crystal. The bands of the groundmass bend around the porphyritic feldspars 
in gentle curves. These feldspars are honeycombed with little drops of 
quartz and flakes of biotite aud muscovite which are often airanged parallel to 
the structure outside. Octahedra of magnetite are visible in the rock; 
microscopic crystals of apatite, rutile, and zircon are abundant. In some 
cases little grains of calcite occur abundantly, even included in the feld- 
spars, and in some localities we find a calciferous gneiss with this same 
structure, in which the groundmass contains a large amount of calcite in 
little grains apparently homologous with quartz and feldspar. This variety 
occurs at several places in the Hoosic valley near the junction between 
the limestone and quartzite, and represents the Hoosac mountain gneisses 
nearest to the limestone. 

At the base of the white gneiss series the rock in many places passes 
so gradually into the underlying granitoid gneiss that it is impossible to 
draw a line between the two. These varieties of the white gneiss are 
very coarse and feldspathic, but the feldspars are white instead of red as in 



HOOSAC MOUNTAIN. 



55 



the granitoid gneiss, and the mica is black. In the shdes the structure is 
essentially the same as that of the granitoid gneiss: the large crystals are 
microcline, broken, faulted, filled with fluid inclusions, epidote grains, 
quartz, mica, etc.; while the groundmass is composed of the usual simply 
twinned feldspars and quartz, mixed with epidote, muscovite, biotite, and 
other minerals. 

At the upper contact of the white gneiss series there are frequent tran- 
sitions into the overlying albite-schists (Hoosac schist); the transition is 
caused by the appearance of bands of mica in the white gneiss alternating 
with bands of feldspar. The latter are often lenticular and composed of the 
simply twinned feldspars which in the schist are proved to be albite. 




Fig. 18.— Metamorphic coDglomorate (Vermont formation). From dump of Central shaft. About one.fourth natural size. 
This is also tlie typical conglomerate. The pebhles are mostly of the fine grained granuUte type. The tine grained 
layers, of which a good example is seen near the top, are composed of quartz grains, hiotite, and some feldspar. They 
represent, of course, sand layers in the original sediment which have undergone considerable metamorphism. 

The last and perhaps the most important member of the white gneiss 
series is the metamorphic conglomerate. This rock occupies a large area 
in the tunnel, occurring on both sides of the central core of granitoid gneiss. 
Nearly all the varieties of the rock are well shown by the dumps of the 
central and west shafts of the tunnel. On the surface it is found on the 
crest of the mountain in the line of the axis of the fold, where the rocks 
have a gentle northerly dip, and measured between conformable contacts 
with granitoid gneiss below and schist above, it has a thickness of about 
650 feet. 



56 GEEEN MOUNTAINS IN MASSACHUSETTS. 

The rock contains pebbles of two varieties : one kind composed of bright 
blue opalescent quartz, the other resembling a fine grained granite, composed 
of quartz and feldspar in small grains, speckled with biotite. These pebbles 
on tlie average are as large as a walnut, though some are much larger, 
and they diminish in size until undistinguishable from the elements of the 
groundmass. The shape is sometimes round, sometimes ellipsoidal, angular, 
or flattened. In Fig. 13, which gives two sides of a large block, the different 
cross-section of the pebbles in two planes is shown. The groundmass or 
cement outside is composed of smaller grains of blue quartz, small feldspars 
resembling the albite of the schist, and biotite and muscovite in large amount. 
The effects of crushing in the rock are evident; the pebbles are often trav- 
ersed by parallel breaks or oblique cracks by which bands of biotite pene- 
trate them, isolating parts of the pebble. Sometimes a pebble is cut in two 
across its axis by such a band of mica. Thus pebbles, in appearance sepa- 
rate, may have been parts of one individual originally. This crushing action, 
combined with the formation of the biotite bands, gives many of these origi- 
nal pebbles flattened shapes, so that they appear as layers of granitic mate- 
rial cut off by the biotite bands in planes oblique to their trend. Some 
varieties of this conglomerate gneiss have a banded structure, due in large 
part to this crushing actioii carried to an extreme. (See Fig. 17.) In some 
cases the pebbles are single crystals of feldspar, and this is occasionally 
microcline. 

Figs. 13-20 show the character of this rock. Some of the pebbles 
consist of fine grained granite containing small grains of blue quartz. Fine 
grained gneissoid layers corresponding to the cement often alternate with 
pebbly layers (see Fig. 18). In some varieties these granite pebbles lie in 
a very micaceous matrix, composed of small feldspars resembling those of 
the schist; in others the pebbles become so small that we get an even banded 
gnejss containing larger grains of blue quartz, the whole forming the ordi- 
nary Avhite gneiss previously described. It is then very difficult or impossible 
to separate the old quartz and feldspar from that formed in situ. The opal- 
escent blue quartz pebbles always retain their round form and are rarely 
entered by the biotite outside. This shows perhaps a connection between the 
formation of the biotite and the feldspar substance. The previous description 
is based on the conglomerate of the tunnel dumps. 



HOOSAO MOUNTAIN. 57 

On the surface here and there conglomerates are found, often associ- 
ated with quartzites; in the latter case the pebbles are all quartz and the 
cement is composed of biotite, muscovite, small feldspar, and magnetite 
crystals. 

On the crest o( Hoosac mountain, in Profile ix, PI. v, the conglomerate 
is represented principally by the finer grained varieties, but toward the base 
the pebbles are much larger and are in part not pebbles, but fragments of 
layers broken up by crushing (see Figs. 15 and 27), giving angular forms. 

When we pass westward from the crest of Hoosac mountain, where 
the conglomerate lies in its normal position, we trace the rock into the 
white gneiss series on the slopes of the mountain. The pebbles have lost 




Fio. 19.— Metamorphic conglomerate (Yermont formation) . Dump, Central shaft. About one-seventh natural size. 

In this variety the pebbles are of much larger size (over 5 inches long), they have the most perfect beach-pebble shape, 
and are composed of a very fine grained granite, which contrasts sharply with tlie much coarser gneissoid cement composed 
of quartz and feldspar grains and mica fl.il£es. The long, white, irregular masses in the center are secondary vein quartz 

their distinctness, and without the favorable exposui-es on the summit and 
from the tunnel we would not suspect their nature; they appear as white, 
flat, lenticular masses of quartz and feldspar, which only in rare places sug- 
gest a conglomerate (see PI. x, b), but when one has traced this rock foot 
by foot into the conglomerate he recognizes the pebbly look at once. It is 
apparent that this change is connected with stretching of the rock, for the 
conglomerate is folded over and then turned under on the west flank of the 
mountain. 

The microscope shows that the quartz pebbles are homogeneous masses 



58 



GEEEN MOUNTAINS IN MASSACHITSETTS. 



of quai"tz, wliicli by optical investig-ation are seen to lia^-e been greatly 
strained; they have a border of l)roken quartz which grades into the ground- 
mass. (See PI. X, A.) They are identical with the blue quartz pebbles 
ofthefossiliferous Cambrian conglomerate (Vermont formation) farther west 
(Clarksburg mountain, Stone hill). The granite pebbles are composed of 
crystalloids of microcline, plates of biotite, and grains of quartz. The micro- 
cline and quartz are crushed and faulted. Veins of a later quartz traverse the 
fissures in the feldspars. Crystals of zircon and apatite and plates of chlo- 




FiG. 20.— Conglomerate (Vermont formation). Crest of lloosat luuuutiiiu south of Sprace hill. 

This sbows a large clifl' of the conglomerate as it occurs in place. The pebbles here are largely blue and white quartz 
and the cement gneissoid. This is in the upper half of the conglomerate horizon. 

rite occur in the feldspar. There are skeleton crystals of magnetite asso- 
ciated with the apatite. The cement is quite similar to that of the white 
gneiss. 

Without here going into the much disputed question of metamorphic 
conglomerates in general, which are found in so many terranes of stratified 
crystalline rocks, ^ it may be said that the reasons for considering this par- 
ticular rock a true conglomerate and not a gneiss containing peculiar con- 

' Cf. A. Winchell, Am. Geologist, vol. 3, pp. 143 and 256. Also C. H. Hitchcock, Am. Geologist, 
vol. 3, p. 253. 



HOOSAO MOUNTAIN. 



59 



cretionaiy forms, are, first, the shape and distribution of these forms (well 
shown in the fignres) and the alternations parallel to the stratification (deter- 
mined by contact with other rocks) of bands of coarse and fine material; 
second, the diverse nature of the pebbles in the same rock (blue quartz, 
white quartz, granulitic rock, granite, etc.) ; and, third, the frequent transi- 
tions in the field into quartzite and quartzite-conglomerate. The production 
of at least part of the mica, feldspar, and quartz of the cement in situ has been 
indicated, and also the efi'ects produced by crushing. 



THE HOOSAC SCHIST. 



The next member of the series is the albite-schist (see Figs. 21, 22, 23, 
and PI. VIII, b), Avhich confoiTnably overlies the conglomerate on top of 




Fig. 21.— Albite schist (Hoosac schist). Dump, Central shaft. Ouetwclfth natural size. 
This is the type with thin Hat quartz layers (the white streaks) and gentle crumpling. 

Hoosac mountain, extending northward for miles into Vermont. On the east 
it extends southward along the east side of the conglomerate and on the west 
in a narrow band along the west slopes of the mountain, curving around so 
as to almost join that on the east. In Hoosic valley masses of these schists 
occur adjoining the Stockbridge limestone and then lying between it and 
the Hoosac gneisses of the Vermont formation. In the tunnel a band occurs 
several thousand feet wide (see PI. v, Profile in) between the west band 



60 GEEEJSr MOUNTAINS IN MASSACHUSETTS. 

of white gneisses and those of the eastern core, and again at about tne cen- 
ter of the tunnel, under the central shaft, they come in east of the con- 
glomerate and fill the eastern half of the tunnel to about 6,000 feet from 
the east portal, where they are succeeded by the silvery -green schists (Rowe 
schists) to the east portal. 

Among the perfectly fresh material found at the tunnel dumps a shiny 
black glistening rock is typical, containing parallel layers of white quartz 
which thin out and disappear in the rock. These flat lenses are sometimes 
very irregular and crumpled by large folds or small puckerings. It is found 
that they correspond to the plane of stratification of the rock wherever the 
schist is seen in contact with other rocks. The black, shiny part of the 
rock is filled with sparkling glassy crystals of feldspar, either in imperfect 
rounded crystals or in simple twins, which contain inclusions of mica, gar- 
net, etc. The basal cleavage planes are sometimes bounded by the brachy- 
pinacoid (M), the prisms T and 1, and the macrodome, etc., but the crystals 
are in general rounded or even angular. 

The feldspar twins are according to the albite law, and the crystal is 
di^'ided into two symmetrical halves, or else the composition-plane is irreg- 
ular, one half taking up most of the crystal, leaving a small strip to the 
other. The rock was powdered and the feldspar, separated by the Thoulet 
solution, analyzed by Mr. R. B. Riggs in the laboratory of the U. S. Greolog- 
ical Survey at Washington with the following result: 

SiOi 69-69 

AljO-j 18-60 

CaO trace 

MgO 0-20 

NaiO 1028 

K.O : 0-40 

Ignition 0-42 

99-59 
COj (Combustion), 0-77^0-44C. 

Basal cleavage pieces with the simple twin give an extinction 4° 
oblique to the twinning-plane and second cleavage (M). Twins measured 
in the goniometer give angles of 172° 46' to 172° 50' between the basal 
cleavages of the two twins. The chemical and physical properties are 
therefore those of albite. These albite cr3"stals vary from large to small; 



HOOSAC MOCTNTAIN. 



61 



they He in planes roughly jjarallel to the schistosity of the rock, but their 
crystallographie directions have no such relation. 

Some varieties of the rock at the shaft are filled with red garnets iu 
dodecahedral crystals. 

The surface rock has the same characters, but with certain variations 
due in part to weathering. The shiny black variety is found here and there, 
but the rock is commonly greenish, indicating a certain amount of chlorite; 
it varies from light to dark green. Garnets are sometimes contained in the 
rock, especially at the base, where a gametiferous horizon occurs. Feldspar 




Fia. 22. — Albite schist (Hoosac srhisti. Dump, Central shaft. One-sixth natural size. 

Here the quartz lenses are more ii-regular and thicker; tlio little white specks dotting the rock are the crystals 
of albite. 

is often present with the garnet. These schists are identical in every detail 
with the schists of Mount Greyiock. 

The por])hyritic albites are prominent in the slide. Simple twins are 
common, but polysynthetic twins rare. Single crystals are common. They 
have a rounded lenticular or flat shape. The groundmass outside the feld- 
spar is composed of rauscovite and biotite, or muscovite alone, chlorite, 
grains and aggregate lenses of quartz, magnetite in octahedra or grains, 
apatite, tourmaline, and rutile. Ottrelite is found in some localities The 



62 



GREEN MOUNTAINS IN MASSACHUSETTS. 



micas of the groundmass bend around the albites in gentle curves (see PI. viii, 
b), and often a band of mica cuts across a feldspar. The albite contains 
inclusions of muscovite, biotite, chlorite, quartz, magnetite, rutile, etc., 
according to their presence or absence in the I'ock. It is common to see 
them in curving bands parallel to the banding of the same minerals outside 
the feldspar. These feldspars evidently crystallized contemporaneously 
with the other minerals in the rock. 




Fig. 23. — Alliite-srbist (Hoosac schi.sl). Dump, Central shaft. About one-oighth natural size. 

Here the quartz lenses are agaiu prominent. It i.s found that they are always parallel to the stratitication. 



The quartz occurs in little grains often arranged in stringers. The mus- 
covite is either in stout plates or is a mass of interlacing fibers or plates — 
the structure characteristic of sericite.,, Biotite and chlorite occur in plates 
or irregular scales; the two minerals occur sometimes side by side in the 
same piece without any sharp boundary between the two, so that the 



HOOSAO MOUNTAIN. 63 

chlorite has the appearance of an alteration product of the biotite.^ When 
the chlorite occurs inde})eudently in stout plates it has a marked pleoclu-oism 
varying fronl green to yellow green, an extinction several degrees oblique 
to the cleavage and twinning with OP as composition-plane. Tourmaline 
and apatite occur in imperfect prisms, magnetite in octahedi'a, and rutile in 
small crystals, often with the heart-shaped twins. 

In several specimens a little ottrelite has been noticed, and at one local- 
ity this mineral occurs in such amount that tlie rock must be called an 
ottrelite-schist. This is interesting in that it still further proves the litho- 
logical identity of the Hoosac, Greylock, and Berkshire schists, since this 
mineral is found in all three of these formations. The hand specimen is a 
shiny, gi-eeuish schist containing crystals of garnet and dotted with little 
black ottrelite 'crystals. In the slide the ottrelite occurs in comparatively 
large crystals with the characteristic indigo-blue, vellow, tdive-green ple- 
ochroism. The extinction is several degrees oblique to the cleavage; it is 
twinned }iarallel to the base, and basal sections give a faint bisectrix. It 
occurs associated with irregular masses of black ore; a number of small 
prisms of ottrelite surround a plate of the ore (ilmenite?). Plates of mus- 
covite and a few grains of quartz compose the rest of the rock. The 
ottrelite is filled with little prisms of rutile with the "knee "-twin. Basal sec- 
tions show the blue color, with vibrations parallel to h (at right angles to 
the axial plane), and the yeUow green parallel to a; hence it has the ^jle- 
ochroism of most ottrelites.^ 

In this schist we recognize no clastic element with certainty and the 
feldspar, quartz, micas, etc., apjiear to have formed contemporaneously, for 
the feldspars c(jiitaiu inclusions of the other elements and in turn are some- 
times crossed bv tongues of mica and qixartz. 

While the term "schist" is applied to this rock owing to its frequent 
coarsely crystalline character, yet its great similarity should be noted to 
crystalline rocks described from Germany and elsewhere as albite-jihi/Uites, 
which contain porphyritic albites with similar inclusions, micas, magnetite, 
etc. 

' This association of biotite and chlorite is common in the hydromica schists of the Green moun- 
tains and is often suggestive of hydration by weathering. 
'^ Cf. Rosenbusch: Physiographie, vol. 1, p. 494. 



64 GREEN MOUNTAINS IN MASSACHUSETTS. 



THE STOCKBRIDGB LIMESTONE. 



The next rock is the hmestone found in Hoosic valley at the base of 
Hoosac mountain and covering the valley west to the base of tlie Greylock 
mountain mass. It occurs in contact with the Vermont quartzite and with 
both the Berkshire and Hoosac schists at several places in the valley. 

The rock is generally a coarsely crystalline white marble banded with 
layers of yellow muscovite or dark graphitic substances, and containing 
layers of bluish quartz. Layers of quartzite are frequent in the limestone 
and the change fi-om one to the other is gradual. Microscopically the lime- 
stone consists of grains of calcite, a few of quartz, flakes of mica, etc. 

It has been mentioned that one variety of the fine grained white gneiss 
often contains considerable calcite, thus forming in some sense a transition 
between the Stockbridge limestone and the Vermont gneiss. A much more 
perfect transition is found between the limestone and Hoosac schist. The 
best case of this kind is found in the "Cove," in Cheshire, where the ground 
is filled with large angular blocks of this rock, which occurs in place in one 
ledge. These rocks resemble a micaceous white limestone filled with little 
dark grains or imperfect crystals of feldspar. In the slide the rock is com- 
posed of a mass of calcite grains, with here and there single grains of 
quartz, or an aggregate of several grains, plates of muscovite and often of 
chlorite and biotite, and large porphyritic feldspar grains in single crystals 
or simple twins, very rarely showing polysynthetic twinning. These feld- 
spars contain inclusions of mica, quartz, iron ore, rutile, and calcite, and 
are in every way identical with the albites of the albite-schists, although 
the exact species of plagioclase has not been determined. The calcite seems 
to play the jiart which the quartz does in the schists: it sends tongues into 
the feldspars, or cuts them in two, and gives one the impression by its in- 
clusions in the feldspar and its occurrence with the quartz and mica that it 
is of contemporaneous origin with the feldspar, mica, and quartz. Rutile 
needles, and masses of ore (ilmenitef) occur in curved bands in these feld- 
spars. Small irregular masses of microcline occur sometimes among the 
quartz grains of the rock. 

On the Greylock side of the valley about 300 yards west of Maple 
Grove station there occur outcrops of a similar feldspathic limestone. Part 



HOOSAC MOUNTAIN 



65 



of the feldspar is here in broad simple twins, but part is microcline in simi- 
lar crystals. The feldspar of this rock needs further investigation. 

The hne-g-rained silvery green or green schists (Rowe schists) which 
occu})y a strip on the extreme eastern border of the map, overlying the albite- 
schist (Hoosac), have not been microscopically investigated by the writer. 

AMPHIBOLITES. 

Last to be described are heavy dark rocks, generally fine grained, in 
which the eye recognizes dark crystalloids of hornblende and irregular 




Fig. 24.— Amiiliiliolitc. Mount Holly, Vermont. 

A band of aiii[iliib()Iitc li feet wide, interstratitied with j^neiss and crumpled with it iu a laj-ge double fold. The 
atructure of the aujphibolite coincides in every detail with that of the gueisa. 

patches of feldspar and cubes of pyrite. In the finer grained varieties the 
rock has a glistening surface due to plates of biotite in films mixed with the 
hornblende, and the rock then has a somewhat schistose structure. They 
rocks have been found in several localities, in all but one case in beds par- 
allel to the structure of the inclosing gneisses and contorted with them. 

These rocks occur abundantly in the Green mountains. The most 

remarkable occurrence is perhaps near Mount Holly and Wallingford, Vt., 
MON xxiii 5 



66 



GREEN MOUNTAINS IN MASSACHUSETTS. 



70 miles north of Hoosac mountain. Here, too, the Cambro-Sikirian 
limestone and Cambrian quartzite (Vermont formation) are succeeded by 
gneissic rocks in the east, which form the central divide of the Grreen moun- 
tains. In the region east of Rutland and directly south of the high mountain 
mass of the Killington peaks there is a marked l)reak in the general topog- 
raphy in an east to west zone, 10 to 15 miles wide from north to south, which 
is characterized by the flat character of the hills. The north to south ridge 
character of the Green mountains is interrupted here, and replaced by gently 




Fig. 25 Amphibolite. Same locality as 24. 

The amphibolite is Interatratified here with quartzite. 

rounded elliptical hills forming an open grazing country. The railroad from 
Bellows Falls to Rutland crosses the axis of the mountains at this place. 
We notice that the soil is colored a deep red and soon find that this is due to 
the decay of masses of these amphibolites, which are interbanded with the 
highly contorted gneisses of the region. Figs. 24, 25, 26 show this very 
well. These bands of rock are parallel to the strata of the gneiss in most 
cases, but here and there send out across the strata tongues which have 
a fine grain at contact and show that these rocks are intrusions. They have 
in general a perfectly parallel sti-ucture, which curves with that of the inclos 



HOOSAO MOUNTAIN. 



67 



iiig gneisses, but also a marked columnar jointing. The form of the hills 
and the very existence of this topographical belt seem due to the rapid 
erosion of these rocks. Their field relations show that they are of intrusive 
origin — dikes, in fact, injected parallel to the strata and then crumpled and 
metamorphosed — and their microscopical characters agree with those of 
similar rocks, described by Lossen, Teale, and many others, which have been 
recognized as altered dikes. They correspond in part to the "metamorphic 
diorites" of Hawes.^ They are briefly described by President Hitchcock 
in the Geology of Vermont, Vol. ii, p. 578, where the remark is make that 
they "may be only huge dikes." 




Fig. 26. — Cruniplert ampliibolite, Mount Holly, Vurmout. Natural size. 

The white bands are feldspar, tlie dark bands hornblende principally. The vertical groovings which coincide with 
the lino of apices of the folds (the specimen standing as in nature) show but faintly in the figure, and are doubtless caused 
by rain tlowing over the vertical surface and following the depressions between the small folds. 

In the hand specimen we see a dark heavy rock, with very faint parallel 
structure in the coarse vai'ieties. Studied in thin section these rocks 
have very uniform characters; the least altered forms, of coarser grain, 
are composed of crystalloids of hornblende and rounded grains of plagio- 
clase feldspar. The hornblende is a massive brownish-green variety in 
short irregular crystalloids, the central parts of which are filled with a dark 
opaque substance, which, with high powers, is resolved into a mass of little 
crystals of rutile; they sometimes inclose crystals of apatite. In some 

' Litliology of New Hampshire, p. 225, 



68 GREEN MOUNTAINS OF MASSACHUSETTS. 

parts of the rock these grains of hornblende fit in between rounded grains 
of a twinned plagioclase. In other places in the rock the hornblende is 
seen to have a narrow fringe of light green pleochroic hornblende (see 
PI. IX, b), massive and not fibrous ; in other grains this entirely replaces the 
brown hornblende, or only little cores of the latter are left. At the same 
time the feldspars in those parts of the rock are filled with small acicular 
crystals of the same green hornblende associated with small grains of pla- 
gioclase, and minute veins composed of these two minerals often cross the 
original feldspars by narrow fissures (see PI. ix, a). The extreme change 
consists in the entire replacement in parts of the rock of the feldspar and 
hornblende by an agg-regate of these small secondary feldspars, with a 
little quartz and epidote in abundance. It is plain that the original plagio- 
clase and brown hornblende has changed to a new plagioclase, green horn- 
blende, some quartz, epidote (taking part of the lime from the feldspar), and 
a little calcite. 

In another form the rock is a fine-grained amphibolite composed of 
crystalloids of bright green or bluish green hornblende, rarely inclosing 
small cores of original brown hornblende, and plates of biotite; both these 
minerals lie in planes, causing the schistose structure. The remaining 
space is filled with little plagioclases which are rarely polysynthetically 
twinned and are filled with grains and prisms of epidote. Grains of 
titanite surround small black cores of original titaniferous iron ore and 
sometimes the titanite grains run out in stringers parallel to the schistosity. 
These feldspars contain, in addition to e^jidote, titanite grains, needles of 
hornblende, biotite flakes, and grains of quartz. In some rocks the little 
prisms have the characters of zoisite instead of epidote. These feldspars 
may occur in broad simple twins like the albite of the schists, or may be 
polysynthetically twinned. The feldspar was isolated from several rocks by 
the Thoulet solution and found to be always plagioclase, generally toward 
the albite end of the series. The hornblende contains titanite and epidote; 
the plates of biotite contain rutile needles. 

A few of these rocks carry irregular masses of red garnet which alter 
to chlorite; they inclose masses of magnetite and green hornblende with 
cores of brown hornblende. The garnet seems to be contemporaneous with 
the feldspar. 



HOOSAC MOUNTAIN. 69 

One vertical dike of this rock at Stamford, Vermont, contains blue 
quartz grains and broken crystals of microcline, which have been taken from 
the country rock of the dike, the granitoid gneiss (Stamford granite). 

GEOLOGY. 

For convenience of description the region covered by the map (PL i) 
may be divided as follows: 

First. The Hoosac tunnel. 

Second. The region embracing the central part of Hoosac mountain 
from the tunnel line on the north to the point in Cheshire where the crest 
of the mountain makes an offset to the west. 

Third. The area covered by the schists occupying the northern and 
eartern parts of the map. 

Fouilih. The region south of Cheshire and of the Hoosic valley. 

Fifth. Hoosic valley schist. 

Sixth. The region around Clarksburg mountain and Stamford, Vermont. 

THE HOOSAC TUNNEL. 

This great engineering work is 4f miles long, entering the base of 
Hoosac mountain from the Hoosic valley on the west, and running in a 
nearly due east direction across the trend of the range. Two shafts have 
been sunk; the deepest, the central shaft, near the center of the tumiel, is 
about 1,000 feet deep, descending from the basin-like depression on top of 
the mountain. (See PL v, Profile iti). The other, the west shaft, is not 
quite half a mile from the west portal, and is 325 feet deep. About 1,000 
feet west of the west shaft, a small shaft called the "well" was sunk, on the 
dump of which specimens of the rock are found. 

The tunnel itself is a large double-track opening, which, starting from 
the Stockbridge limestone at the west portal, passes through all the rocks 
of the series at least once. But several tilings combine to greatly lessen its 
value as a geological section of the core of the mountain. A considerable 
proportion of the tunnel is now bricked over, and only in the manholes, 
every 250 feet, can the rock be seen; and secondly, the covering of soot 
and smoke on the rock is very thick, making it necessary to get fresh sur- 
faces by hammering. The difficulties of working by lamplight in the smoke 



70 GREEN MOUFTAIlSrS OF MASSACHUSETTS. 

of passing trains are also considerable. Moreover, that part ot the tun- 
nel wliich would have afforded the most important contact for determining 
the relations of the Stockbridge limestone to the Hoosac mountain rocks is 
entirely bricked over; it lies in the decomposed rock which caused so much 
trouble during the building of the tunnel. Therefore, while the general 
distribution of the rocks is easily found in the tunnel, much less was done 
in the way of determining relations by contact than would have been possi- 
ble under more favorable conditions. 

In the following description the reader is referred to Profile in, PI. v. 

Starting at the west end of the tunnel we find the Stockbridge lime- 
stone of Hoosic valle}^ in the long open cut which leads to the tunnel 
mouth, and passing under the masonry of the portal ; the dip alternates in 
a series of small folds, sometimes east, sometimes west. From the portal 
for 2,700 feet the tunnel is bricked, but at several of the manholes we find 
rock in place. At a little over 1,600 feet we find in a manhole the first 
'occurrence of the fine-grained variety of gneiss with small porphyritic feld- 
spars, and the same rock again at about 1,900 feet in. Near 2,000 feet the 
albite-schist (Hoosac schist) is found in all of the manholes to about 3,800 
feet. Then by transitional rocks this passes into the white gneisses which 
extend to 6,000 feet, where by gradual transition they pass into the coarse 
granitoid gneiss; this rock runs as far as 10,500 feet, then after 250 feet of 
bricking the conglomerate-gneiss is found at 10,770 feet, and this extends 
to 12,100 feet, where the albite-schist series is found in conformable con- 
tact with the conglomerate-gneiss. The albite-schist, succeeded by the 
Rowe schist, is then found through the rest of the tunnel. We find then 
in the tunnel, going in from the west: first the limestone, which extends 
into the tunnel proper a short distance, but is now entirely bricked in; 
then the fine grained, banded, white gneiss (Vermont formation), extending 
to about 2,000 feet from the portal; then the albite-schist for 1,750 feet; 
next the white gneiss (conglomerate-gneiss) series (Vermont) for a little 
over 2,000 feet; then the granitoid gneiss (Stamford gneiss) for a little over 
4,000 feet; then white gneiss-conglomerate for 1,500 feet; and the schist 
formation (Hoosac schist overlaid by Rowe schist) for the rest of the way, 
or about 12,900 feet, of which the last 6,000 is occupied by the gi-eenish 
sericitic or chloritic Rowe schist. 



TT S.G>:ni,OGIGAJ- SUHATTi'. 



MONOOnATU 5XIIL pl.t: 



GEOLOGICU^ PROFILES 0¥ HOOSAC MT. 

EAST- WEST PROFILES. 




\. Northern Section Hoosac Mi 



H.Section from Natural Bridge through Schist ndge 




W •:^LJ'ortat, 



Wh 6n Schist. iVhife Gn Granitoid Gnoiss Can^hmeratS AlbitB Sc/iist. 

IS.. Hoosac Tunnel ant^ Hoosac Mi. 



ffar\-e Schist 



W. Sect/on running upl^t CreekS of Tunrrel Line 
to Spruce Hill. 



" Buttress 

TJ.^Sect/on through Buttressto crest Hoosac Mt. 



Hoosac Mt 





<S5 



'W. Section across Hoosac Mt throu_gh contact on Pond. 




Clint set Conform 
Within 20 ft Gorjtaei 

^m.Sect/on up 51^ CreekS. of Buttress. 



VilL, Bowen's Creek Section. 



Lineof Tunnel Spruce Hill Contact Contact 



Horizontal and Vertical Scale 3000 ft = Imch 

\ \ Limestone (Stockbrid^e) 

□ Rowe Schist 

IZZl Albite Sciiist (Hoosac Schist) 

I 1 White Pnelss quartzite conglomerate 

■"■ (Vermont Formation) 

[ I Granitoid Gneiss (Stamford Gneiss) 

I Pitch ofaxes- 

The sections are arranged in meridian. 

LONGITUDINAL OR NORTH SOUTH PROFILES. 




Tunnei643oftfromWPortai 'S.. LonPitudinal section Hoosac Mt. from Bowen's Creek 

at about W contact Granitoid Gneiss 

and Conglomerate. tO line of TuOnel. 



^.r^:^^^ " 



'Xl.SrN section through end synclinal 
"canoe"point Hoosac Mt. 



'^.NrS.measured Stadia Section from contact Granitoid 
Gneiss and Conglomerate to Spruce Hill Fla^ 

With dip of 20' thickness Con^iomerate 740 ft 

" - "IS" ■' " 600 n. 



IKK. Section through extreme length Granitoid Gneiss. 



A HOf-Na. CO UTH aMTO.MD. 



HOOSAC MOUNTAIN. 71 

As regards the structm-al observations it was not practicable to attempt 
these in detail; in the first or westerly band of white gneiss, found only in 
manholes, both east and west dips were observed, and no contact was seen 
with the next rock — the albite schist. 

This next band, the albite-schist, has in general an easterly dip, but 
towards the contact with the next band of white gneiss has a very steep dip 
varying from east to west. There is a conformable contact and transition 
between the two r<:)cks. 

In the next band of white gneiss dips were noted varying from steep 
east to west: the observations are put down in the section. At about 6,000 
feet the rock becomes coarser in character, corresponding to the white 
gneisses, transitional to the granitoid; it contains frequent round jjebbles of 
blue quartz, corresponding to the conglomerate found in the dumps of the 
tunnel. From here for about 700 feet we have transitions to the coarse 
gneiss; lenses or layers of fine-grained g-neiss are frequently seen. Nearly 
a whole day was spent here in searching for a contact, by careful hammer- 
ing, but none could be found; there is an evident transition, as observed 
elsewhere at points outside the tunnel. 

The area of the coarse granitoid gneiss contains rock of an even char- 
acter; whatever structure exists by arrangement of mica planes, etc., 
remains flat or gently rolling east and west. The east contact between this 
I'ock and the conglomerate-g'ueiss is concealed by the brickwork. 

This east band of the conglomerate-gneiss, as on the surface, is char- 
acterized by a steady, well-marked easterly dijj of 20° to 30°, and this 
ends very near the central shaft, where the rock is overlain by the albite- 
schist; its thickness is accordingly about 600 feet, which agrees closely 
with that found on the surface. The structural planes of the two rocks are 
absolutely conformable, both dipping east about 25°. The line of contact 
is easily found ; witlun a few inches of rock they pass into each other with- 
out a break. From here through the rest of the tunnel only the albite- 
schist, passing in the last 6,000 feet into fine-grained greenish schists, is 
found. The dip of the structural planes is always steep east. The rock 
varies in character as on the surface, in color, coarseness, amount of albite, 
quartz lenses, etc. 

The main facts then brought out in the tunnel are that there is a, large 



72 GREEN MOUNTAINS IN MASSACHUSETTS. 

central mass of coarse granitoid gneiss (Stamford gneiss) forming the core 
of Hoosac mountain; that this is flanked on either side by a band of tlie 
white gneiss-conglomerate (Vermont formation), the eastern band having a 
steady east dip and conformably overlain by the albite-schist series, the 
western band being broader, with varying dips passing by gradual transitions 
into the coarse gneiss, and bounded on the west by a narrow band of the 
albite-schist (Hoosac schist) ; the contact between the two rocks being con- 
formable and transitional. The schist bandis succeeded on the west by another 
band of fine-grained white gneiss (Vermont) and this in turn by limestone 
(Stockl)ridge), no contacts being observed. We shall speak of this anti- 
clinal structure further, after describing the geology of the surface of the 
mountain. 

THE REGION EMBRACING THE CENTRAL PART OF HOOSAC MOUNTAIN. 

The map shows the distribution of the formations in this area. The 
central part, forming the crest of the mountain, is occupied by a long irreg- 
ularly oval area of the granitoid gneiss, the long axis of which runs nearly 
north and south parallel to the trend of the mountain, with a length of 6 
miles and a width at one place of 1^ miles. This is surrounded by a zone 
of the Avhite gneiss series (Vermont) about one-half mile wide, which at 
the southern end of the granitoid gneiss core expands into a broad area of 
white gneiss-quartzite, extending down to the southern border of the map. 
To the east, the great expanse of the albite-schists (Hoosac schist) borders 
the zone of the white gneiss-conglomerate, running in an almost straight line 
along the whole eastern edge of that formation to the southern edge of the 
sheet. It circles around this formation to the north, forming the surface 
rock in the whole northern part of the Hoosac mountain, and sends a long 
narrow tongue down on the west side of the white gneiss zone, which bends 
around with this at the southern end of the granitoid gneiss area, and 
.becomes gradually thinner until it can be only doubtfully traced by loose 
blocks at the extreme point of the curve. 

Lying west of this tongue of Hoosac schist we have another area of 
fine grained white gneisses or quartzites, with a vai-iable width, which dis- 
appear under the drift a little north of the tunnel, and at the south join 
the great mass of white gneiss at the southern end. 



HOOSAC MOUNTAIN, 



73 



Finally the limestone borders this last area on the west. The relations 
of these rocks — granitoid gneiss, white gneiss, and metamorphic conglom- 
erate — are best shown at the extreme northern end of the area of the first 
rock (see Profile ix, PL v) on the crest of Hoosac mountain. The granitoid 
gneiss is here of the typical variety, with bright blue quartz and a structure 
well marked in the mass. This dips about 10° to 15° a little east of north. 
In a little north and south cleft, just south of a small swamp, we find this 
rock in contact with the overlying conglomerate gneiss. Fig. 27 shows 
this. The series dips 20° in a direction north 25° east. The lower part 




Fig. 27. — Contact of granitoid gneiss (Stamford gneiss) and metamorpliic conglomerate (Vermont formation). Top of 
Hoosac mountain. S(»utli at Spruce liill. 

The contact runs from the left band lower corner to the right hand upper corner. This conglomerate is also shown 
in Fig. 15. Notice that the lines of structure of the gneiss follow conformably those of the conglomerate. 

of the exposui'e is formed of typical granitoid gneiss. Upon this the 
lower beds of the white gneiss-conglomerate rest conformably. In the latter 
rock it is apparent at once that crushing has largely affected the form of the 
pebbles. To this cause their flattened character and truncation by oblique 
planes of mica is undoubtedly due, and yet they are in large part pebbles. 
Not only their general shape, but the lithological distinctness shows this. 
They are composed either of massive white quartz, or blue quartz, or smoky 
quartz, or in some cases of a white granulite, or lastly of a fine gi'ained white 



74 GEEEN MOUNTAINS IN MASSACHUSETTS. 

gneiss containing blue quartz and biotite. In one large pebble the gneissoid 
structure in it is quite oblique. It is easy to see that this conformity of con- 
tact in these two rocks, both of which have so much secondary structure 
developed in connection with crushing, may be due to the crushing itself 
From this contact northward the crest of Hoosac mountain makes a sharj) 
rise in a series of bluffs facing south, the top of each bluff sloping gently 
to the north. Profiles A and b, PI. xi, show this feature well. In PL xi, b, 
we are looking west; in the hollow at the extreme left is the contact 
spoken of, and the white gneiss-conglomerate extends to a point shown 
about the middle of the picture, and is then succeeded by the albite-schist. 
The gentle northerly dip of the whole series can easily be seen by the 
slope of all the steps of the crest to the north (right). See also PI. v, 
Profile IX. Stalling from the granitoid gneiss at the base we find a 
thickness of 600 to 700 feet of this white gneiss conglomerate with a very 
steady northerly dip of 15° to 20°. At the base the rock is quite coarse, as 
pre^'iously described. As we ascend in the series it becomes gradually finer 
grained. The granulite-gneiss pebbles become smaller and smaller and are 
more frequently crossed by the mica of the groundmass; the quartz pebbles, 
and especially those of blue quartz, preserve their rounded character. Fig. 
20 (from a large cliff of this medium grained rock) shows this character 
finely; the large pebbles are mainly of blue quartz^ As we go higher up 
the rock becomes more and more even grained until we get a finely banded 
muscovite-biotite-gneiss. In many places the conglomerate is finely crum- 
pled or fluted, the axis of these foldings gently inclined, parallel to dip. PI. 
X, B, shows this character; here the flattened lenticular masses we call pebbles 
are themselves gently folded with the rest of the rock. At the top of the 
conglomerate this rock is overlain conformablv by the Hoosac albite-schis. 
series. At a distance of half a foot from the latter, thin bands of extremely 
garnetiferous Hoosac schist alternate with bands of the fine grained con- 
glomerates, forming a well marked transition. The rock at the base of 
the Hoosac schist group is extremely garnetiferous and of a dark, almost 
black, dense character, with little feldspar. This garnetiferous character at 
contact with the white gneiss is almost constant in this region and seems to 
extend some distance above the contact, perhaps 50 or 100 feet; in the space 



HOOSAC MOUKTAIN. 75 

covered by our Profile ix, PI. v, (which is plotted from a stadia section, 
checked by triangulation) it will be seen that there is nearly 800 feet of the 
albite-schist to the summit of Spruce hill, where the section stops. The 
schist preserves its gentle northerly dip throughout, with a quite uniform 
character, often very rich in the albite crystals. 

The profile we have just described gives us the key and starting point 
for the geology of Hoosac mountain. As will be seen later, this profile is 
taken at the northern end of the overturned anticlinal axis of Hoosac moun- 
tain, tlie whole axis having this gentle pitch or plunge to the north which 
causes the dip of 15° to 20° northerly. The granitoid gneiss disappears 
at the surface here and is found again in the center of the Hoosac tunnel 
in the same meridian line, but 1,400 feet lower in level. Although the north- 
erly pitch of the axis has here brought the to]) of the arch of the granitoid 
gneiss far below the surface, enough of the arch remains above the tunnel 
to allow a length of sevei'al thousand feet of the excavation to lie in this 
rock. (See Profile x, PI. v.) 

Now going back to the contact of granitoid gneiss and gneiss-con- 
glomerate at the south end of Profile ix, PI. v, and tracing the contact of 
the two rocks westward, in a few hundred feet we come to the extreme west 
prest of Hoosac mountain overlooking the valley (see PI. iv). Here we find 
the continuation of the two rocks in contact again with the same strike and 
dip. The granitoid gneiss runs a hundred yards north and then disappears; 
the white gneiss-conglomerate makes a sharp turn over the prong of the other 
rock and comes in on the west side of it, on the slope of the mountain; the 
white gneiss strikes north 40° east and dips 50° west, instead of striking north 
67° west and dipping northeast. The manner in which one rock mantles 
over the other can be seen very plainly; at the turn they are within 20 
feet of each other. The successive outcrops of the white gneiss on lines 
radiating out from this point of the turn show the same curving around of 
the outcrops from a northwest to a northeast strike. 

Following in the same way the top of the conglomerate towards the 
west, we find it strikes northwest until the extreme west crest of the moun- 
tain is reached, closely overlain by the Hoosac schist; the outcrops then 
suddenly turn and descend the slope of the mountain obliquely in a north- 



76 GREEN MOUNTAINS IN MASSACHUSETTS. 

west direction, followed closely by the overlying schist. The rocks here are 
very much crampled; the axes of the crumplings have a steady direction 
about north 10° east and a gentle northerly inclination of 10° to 15°, 
while the actual line of outcrop runs northwest down the mountain. In this 
way the schist mantles over the conglomerate and follows it down; grad- 
ually the line of outcrops turns and runs nearly straight down the moun- 
tain until the extreme point is reached nearly half way down to the valley. 
Here we find the gneiss striking north 10° east and pitching 10° to 15° 
northerly, very much crumpled, and passing under a cliff of the schist like- 
wise crumpled. The two rocks are here again connected by transitional 
layers in which bands of white gneiss alternate with bands of schist, and 
the gneiss contains a great abundance of the porphyiitic feldspars; the 
schist is also the dark garnetiferous variety. At this point the same change 
of position previously described occurs, namely by a sudden turn, which 
can be traced by connected outcrops; the scliist comes in on the west side 
of the white gneiss with a strike north 10° east, a strong northerly pitch, 
and a dip of the foliation (very much crumpled) generally steep west. 
The reader is referred to the map (PI. iv) for the graphic presentation of these 
facts J the outcrops have been carefully traced step by step and important 
points located by placing flags in the trees and putting in the points by the 
plane table. Note how closely the apices of the turn in the granitoid gneiss 
and white gneiss coincide, showing the conformity of the series. After the 
rocks have made this turn so that the overlying formations come to lie suc- 
cessively west of one another, there is no difficulty in tracing their course 
to the south along the side of the mountain. From the turn in the contact 
between granitoid gneiss and white gneiss-conglomerate, the line of contact 
runs obliquely down the mountain in a south by west direction for about 2 
miles, where it reaches its lowest point topographically; the actual contact 
has not been found, although the two rocks commonly come close together, 
but talus from the granitoid gneiss conceals the contact. The white gneiss 
often forms a flat bench 100 or 200 feet wide. The structure of the white 
gneiss, as mentioned before, dips very steeply west just after the turn; 
within a quarter of a mile it is found to dip near the contact with the gran- 
itoid gently east, from 10° to 25°; but commonly the i*ock is greatly crum- 



HOOSAC MOUNTAIN. 77 

pled, the axis of the crumples running noi-th 20° east and having a strong 
northerly pitch. Profile iv'', PL v, shows this feature. 

In the same way the contact between the white gneiss and the band of 
Hoosac albite-schist can be traced south from the point where we left it. 
Both rocks are very much crumpled, the axis of the crumples striking a 
little east of north and strongly inclined to the north; the contact can be 
found within a few feet; the structure of the two rocks in the large cliffs 
can be seen on the average to be nearly perpendicular or dipping steep 
west. The schist near the contact is the dark garnetiferous variety found 
at the base of that rock on the top of the moimtain. 

As will be seen from the map the schist forms only a narrow band, 
bordered again on the west by another area of gneiss. 

We will now take up the relations of the granitoid gneiss and white 
gneiss-conglomerate and trace them around from the point where they were 
last seen at the tui'n. As said above the line runs obliquely down the moun- 
tain side, the structure of the two rocks dipping gently east; that is to say, 
the white gneiss dips in under the granitoid instead of overlying it. Near 
what is marked Southwick creek on the map the granitoid gneiss reaches 
its most westerly extension and its lowest topographical level, and from 
here the outcrops begin to rise and to turn gradually and run southeast. 
PI. v. Profile VII, which runs up Southwick creek, shows this relation well; the 
white gneiss has a steady flat moderate easterly dip carrying it under the 
granitoid gneiss. At about this point we notice a transition from the white 
gneiss to the granitoid; the white gneisses are coarse and very feldspathic, 
so that it is almost impossible to find any definite line of demarkation 
between the two rocks. Continuing a third of a mile south from Southwick 
creek we come to the place where Profile x, PI. v, crosses the contact of the two 
rocks. The actual junction of the two rocks is found here in so far as there 
can be said to be a junction. The strike is north 40° west and the dip 15° 
east. Within a hundred feet horizontal the rock forms a transition between 
the coarse typical granitoid gneiss on one side and the fine-grained banded 
white gneisses on the other. From here the contact tm'ns and ascends the 
m6untain rapidly, the coarse transitional gneiss making it always impos- 
sible to find any exact contact; the strike is north 25° west and the dip 



78 GREEN MOUNTAINS IN MASSACHUSETTS. 

flat east. After reaching the crest of the mountain the Hne of contact turns 
approximately north and south with north and south strike of the structure 
of both rocks, the dip of the structural plane is rolling and often is west- 
erly. When we come to the extreme end of the west side of the granitoid 
gneiss area, where the line makes a sharp turn to the east, we find well 
marked in both rocks and in the transitional forms a strike nearly due east 
and west and a rather gentle northerly dip (strike north 77° to 85° west 
dip 10° northerly). The coarse transitional rocks belonging to the white 
gneiss series can be traced to the round spur about 1 mile north of Savoy 
Hollow, where by a sudden crumpling the rocks turn around to a north to 
south strike and an easterly dip and then run northward. 

If we go liack to the contact of the two rocks first described (south of 
Spruce hill), and follow it east, we find that the line of contact preserves its 
east and west strike for half a mile and then begins to turn southerly. The 
congloraei'ate preserves its character fairly well for that distance; but half a 
mile further the strike is about north and south or north 10° west, showing 
considerable vai'iations, but there is always an easterl}^ dip of 20°. The line 
of contact here turns southerly and is concealed by drift. Half a mile farther 
south we find the coarse transitional gneiss, instead of the conglomerate, 
striking here north 42° west and dipping 45° east. For three-quarters of 
a mile this rock continues until we come to the shore of the second pond 
crossing- Profile v, PI. v. Around the shores of this pond the relations of the 
rocks are well exposed. On the west shore the typical granitoid gneiss occurs 
with blue quartz, witli a north to south strike and easterly dip of the 
structure. For 1,000 feet east of here we have a series of outcrops, partly 
in the water, which consist of the coarse transitional gneiss, often contain- 
ing granulitic lenses that resemble the pebbles of the conglomerate. 
There are many loose outcrops of the genuine conglomerate with blue 
quartz, granulite, and gneiss pebbles, which make it very probable that 
ledges of this rock exist here. Half way across the pond we find the con- 
tact of these coarse transitional gneisses with the Hoosac albite-schist, the 
latter resting on the gneiss and the structure of the two rocks absolutely 
conformable — strike north 10° east, dip 25° easterly. The schist is very 
garnetiferous, as usual near the contact, aud covers the rest of the 



HOOSAC MOUNTAIN. 79 

sheet from the contact east to the Rowe schist. The area covered by these 
transitional coarse gneisses therefore occupies the geological position of the 
conglomerate-gneiss, a fact which the occm'rence of the "loose ledges" of 
conglomerate seems to confirm. North of the lake the continuation of this 
coarse transitional gneiss is found at intervals with the same strike and dip. 
From here for 2^ miles south the place which should be occupied by 
the white gneiss-conglomerate is covered with drift, and not a single out- 
crop is found. The albite-schist, with a constant north to south strike, 
borders on the east and the granitoid gneiss on the west. Opj^osite the 
post-office of Savoy Center the next outcrop is found. This is quite con- 
glomeratic in aspect, with round, blue quartz or granulite pebbles, and a 
strike north 15° west and dip 45° east. Intervening between this and the 
typical granitoid gneiss to the west we find the same coarse transitional 
gneiss, with somewhat varying strike and dip. Continuing south from this 
last exposure, on the road leading to Savoy hollow, we find occasional 
outcrops of coarse transitional gneiss with the same nearh* north to south 
strike and easterly dip. This brings us about to the extreme point of the 
ai-ea of granitoid gneiss and to the white gneiss-conglomerate band fol- 
lowed around from the west side. The relations of the rocks at this point 
are peculiar and deserve a special description. The to])ography here is 
well marked. It is easily seen on the map that a long spur runs out from 
tlie pt)int of the granitoid gneiss for a mile and more toward Savoy hollow. 
This spur is caused by the meeting of the white gneisses of the east and 
west areas, those on the east coming down with a north to south strike and 
easterly dip, those on the west striking across with a nearly east to west 
strike and northerly dip. We find on the spur the rocks very sharply 
crumpled, representing the sudden turn of strike and dip; some layers 
striking east and west can be traced to the place where the-\' curve around 
and run southerly with a steep easterly dip. At one point on the spur, 
about a mile north of Savoy hollow, we find a curious curving series of 
outcrops of a very coarse porphyritic gneiss, containing large rounded 
feldspar crystals, blue quartz, etc. — an "Augen" gneiss. The outcrops on 
the east side strike north 5° west and dip about vertically. This gradu- 
ally curves around to an east and west strike and steep southerly dip, then 



80 GEEEN MOUNTAINS IN MASSACHUSETTS. 

to a northwest strike and nortlierl}' dip — that is, the layers circle around 
in the space of a few hundred feet, giving a canoe-shaped fold. The 
development of the very large porphyritic feldspars just in the turn is also 
significant. In short, this space, so marked topographically, is the place 
where part of the layers of the white gneiss are crumpled and pinched 
together in the extreme point of the great fold which we have been describ- 
ing. It Avill be seen from what has been said that the central part and 
crest of Hoosac mountain is composed of a great anticlinal fold in the three 
members of the series — granitoid gneiss (Stamford gneiss), metamorphic 
conglomerate (Vermont formation), and albite-schist (Hoosac schist) — and 
that this fold has a pitch or inclination of its axis of 10° to 15° to the 
northward, while the western side has been pushed in under or overturned, 
this overturn continuing into the southwestern part. The beds are in 
inverted order on the west and southwest sides; in normal order on the 
north and east sides. By reason of the pitch of the axis the same rock 
occm-s in the tunnel, 1 mile north of the last appearance of the granitoid 
gneiss on the surface, flanked on both sides by the conglomerate and 
albite-schist; these two formations on the east side dipping east, overlying 
the granitoid gneiss in normal order On the west we find the same transi- 
tions between granitoid gneiss and white gneiss-conglomerate that were 
observed on the surface, and a nearly vertical structure. Profiles x and 
XII, PI. V, give these relations graphically. 

The belt of Hoosac schist which is seen on the map to run around 
the central gneiss and nearly to join the great mass of schist on the east, 
starts off" from the main mass as a bi'oad tongue, narrowing rapidly to a 
small constant width. At various points its top and bottom contacts with 
the gneiss on either side have been observed. Over the tunnel this schist 
can not be found in definite contact with the western gneiss; on the con- 
trary, there is a gradual transition, which can be seen in the outcrops on 
the slope of the mountain above the Avest shaft. We hardly find here in 
the schist what we can call a dip of any kind — simply the usual fluting, 
with the strong northerly pitch of the axes. Following the band down to 
a point some hundred yards north of Profile iv^, PI. v, we find here the east 
contact of the schist and white gneiss. The schist is very garnetiferous, as 



V S GEOLOGICAL SURVEY 



MONOGRAPH ZXm PL, VI. 




AHni\diCD i;rK a.MW.Mr 



HOOSAC MOUNTAIN. 81 

elsewhere. Both rocks have their structure vertical with the small folds, 
pitching 10° to 15° northerly. In Profile iv*, PI. v, itself we get another 
contact. From here for 2^ miles to Profile v, PI. v, the black schist is con- 
cealed; then outcrops occur with easterly dip; east and west contacts with 
the gneiss are concealed. In the creek of Profile vii, PI. v, we have a long 
series of outcrops of the schist with the easterly contact beautifully shown, 
the westerly within a few feet. These schists are extremely crumpled, as 
shown by the quartz lenses; these crumples pitch gently northerly. The 
rock is very garnetiferous near the eastern contact with the white gneiss; 
in other places feldspathic. At the east contact we have the white gneiss 
dipping 20° easterly; it is a white, muscovitic variety. The schist layers can 
be seen within less than 4 feet of strata from the base of the gneiss, dipping 
gently under it; intervening ledges are covered by the water or soil. It 
indicates perfect conformity, both series dipping east. After forming a 
series of cascades over this schist the creek runs out on a level and we 
find here the rock succeeded by outcrops of micaceous quartzite or fine 
grained gneiss, with same strike (north 10° east) and dip 25° east; the dis- 
tance covered from one rock to the other is 25 feet horizontally. 

For half a mile south from the upper contact of Profile vii, PI. v, it 
can be traced very closely with the same strike and gentle easterly di]^, 
the contact being found often within a few feet and the structure of the two 
rocks being conformable. At a mile from this contact we come to Profile x, 
PI. V. Here the actual contact was again easily found in the rocky cliff, 
both white gneiss and black garnetiferous schist much crumpled, but with a 
general easterly dip of 10° to 15°. The strike is north 25° to 30° west 
and the small crumples pitch northerly 10° to 15°. This inclination affects 
the topography; Fig. 10, p. 43, represents this spur, in which the gentle 
slope towards the left of the picture (north) is due to the pitch of the rocks. 
The lower contact is not found here. In Profile viii, PI. v, we have this 
schist again outcropping, but neither contact. 

A mile farther, on the north fork of Tophet creek, in a deep gorge, we 
find fine exposures of the schist, much crumpled, and at the head of the 
gorge its contact with the overlying white gneiss, which here again con- 
tains transitional layers of micaceous gneiss. Both strike north 10° west 

MON XXIII 6 



82 GREEN MOUNTAINS IN MASSACHUSETTS. 

and dip 15° easterly. From this contact the line of the two rocks is easily 
followed over the hills to the south fork of Tophet creek, 1 mile. Here, after 
crossing numerous exposures of the western band of gneiss, the creek falls 
over cliffs of the typical albite-schist with same strike (north 10° west), and 
gentle easterly dip under the overlying white gneiss, with which it is con- 
nected by transitional beds as before. The schist is always garnetiferous. 

From here for half a mile the schist can only be traced by loose pieces 
and one outcrop until we reach the corner of the mountain; here we find 
it again in place and the contact vrith the overlying gneiss is within 2 feet 
of strata. Both are conformable in structure and strike north 45° west dip- 
ping 25° northerly. Here both rocks are turning to assume their east to 
west strike at the extreme point of the turn (curve), and only crumpled out- 
crops of schist are found, as is usual in these turns. 

Following the schist east one-half mile we find it overlying the west- 
ern band of white gneiss, which has here curved around so as to lie south; 
the upper contact is not seen; the schist is garnetiferous and passes into 
the underlying white gneiss by micaceous layers. The strike is north 75° 
east, dip 25° northerly, and one-quarter mile farther east cliffs of the gar- 
netiferous schist are found striking east and west and dipping 20° north, 
closely and conformably underlain by the southern band of white gneiss. 
From here for a mile only fragments of the schist are found. Within a 
quarter of a mile of the extreme turn a small outcrop of feldspathic schist, 
exposing a thickness of 30 feet, is interstratified with the iine-grained gneiss; 
strike north 40° west, dip 20° north. One-half a mile farther in the line of 
the strike of the gneisses, which are curving at the extreme point from an 
east to west to a north to south strike, a solitary outcrop of garnetiferous 
and feldspathic schist is found, with a vertical dip and strike north 10° 
east, which represents probably the last trace of this tongue, which we 
have followed continuously from the main mass. It seems to be squeezed 
out in the folds of white gneiss. 

We come now to the band of gneiss (Vermont formation) lying west 
of this band of Hoosac schist. All of this gneiss follows closely the schist 
around to the extreme southeast point, where it merges into the great area 
of gneiss in the southern part of the map. The gneiss of this area has a 



HOOSAC MOUNTAIN. 83 

uniform and peculiar character; it belongs to the fine grained porphyritic 
gneiss already described and has a tendency to pass into micaceous quartz- 
ite or even pure quartzite. 

The first exposure found is on the side of the mountain about 1 mile 
north of the tunnel line, where it is within a few feet of the albite-schist, 
which here extends up the mountain. Both rocks are conformable, strike 
north 30° east, dip 60° east; to the north the rock is covered with glacial 
drift, so that it is uncertain where it finally disappears, but the two bands 
of albite-schist come close together both east and west of it. This rock 
shows a remarkable tendency to disintegrate. This "rotten gneiss" caused 
great expense and loss of time in building the western part of the tunnel. 
At the tunnel line outcrops of this rock are found on the surface at the 
west shaft and on the mountain above for over 100 feet, when they are suc- 
cseded by the schist; but transitional rocks made it impossible to draw a 
line. Toward the west edge of this gneiss band, a few hundred yards north 
of the tunnel line, an old iron mine alongside the road is composed of a 
massive quartzite containing masses of limonitic iron ore, the structure of 
which is not determinable. This gneiss was also found in the tunnel at 
several manholes, and in the creek just south of the tunnel line we find 
several outcrops of this rock as indicated on the map, all striking about 
north 20° east and dipping east at varying angles. Also a few hundred 
yards south of the portal of the tunnel we have an outcrop the strike of 
which would carry it very close to the portal. 

When we come to the sharp little hill of Profile iv" ("the Buttress") 
we have fine exposures of this gneiss (see Plate v). It is plain, from this 
section, that in this band of gneiss we have considerable folding. One 
sharp anticlinal is plainly shown here with many smaller crumples. There 
are several hundred feet of covered space between the western outcrop of 
gneiss and easternmost of limestone, but the contact with the schist is very 
close. The folds of this gneiss have a strong northerly pitch of as much 
as 10° in Profile iv". 

From Profile iv'' for 1^ miles to Profile vii we have only two or three 
scattering outcrops of this rock (see PI. v). At Profile vii it is represented 
by one outcrop of micaceous quartzite closely underlying and conformable 



84 GREEN MOUNTAINS IN MASSACHUSETTS. 

to the overlying schist; strike north 10° east, dip 30° easterly. Three- 
quarters of a mile south, in the next creek, two or three outcrops of mica- 
ceous feldspathic quartzite strike north 10° west, dip 25° east. The curve 
of the strike has begun here. 

Broad benches strewn with glacial drift cover this rock in all this 
part of the mountain. At this place, opposite the north part of the town 
of Adams, the line of junction of the limestone with the gneiss band seems 
to make a curve westward, for we find one outcrop of this gneiss in 
a small quarry near Adams. The strike is north 10° west, dip 25° east. 
A few hundred yards south, in the creek marked Anthonys creek, we get 
outcrops of a similar gneiss; strike north 8° west, dip 50° easterly. Below 
this a few feet we find a series of outcrops of a massive micaceous quartz- 
ite, the bedding of which dips 25° to 30° easterly and strikes north 15° 
west. A little lower down along the road we find the Stockbridge lime- 
stone striking north 15° west, dip 25° east; we find this within a few feet 
of the quartzite along the road. Then in the bank there is a crumbly 
transitional rock between the limestone and quartzite, so that the Stock- 
bridge limestone and this quartzite seems to form the same rock, and the 
fine grained banded gneiss appears to overlie the quartzite. 

In the canyon of Tophet creek we have clifi"s of the limestone with 
varying strike and dip. Ascending the creek, near the upper edge of the 
canyon, we find a large ledge of massive vitreous quartzite which strikes 
northwest and is overlaid by large loose ledges of the fine grained gneiss, 
striking north 35° west, dip east 50°. Several hundred feet along the strike 
south, and in the creek bed there is the conformable contact of a small 
piece of massive quartzite overlaid east by the gneiss, both dipping east 
and striking north 10° west. Still farther south on Tophet creek, near the 
entrance to Bowens creek, there are extensive ledges of the fine grained 
gneiss striking north and south and dipping east. It is therefore evident 
that this rock, underlain to the west by a massive quartzite, is succeeded 
by the limestone, and that the limestone and quartzite pass into each other 
by transitions. In the canyon of Tophet creek this contact is concealed; 
it is some hundred feet from the quartzite to the first cliff' of limestone. 

I'or 2,000 feet east across the strike from the fine grained gneiss at the 



HOOSAC MOUNTAIN. 85 

entrance of Bowens creek into Tophet creek, a gently sloping bench con- 
ceals all outcrops; then in Bowens creek we have Profile viii giving us a 
typical section through this band of gneiss, the rock varying between a 
vitreous quartzite, micaceous quartzite, and the fine grained gneiss typical 
of this area. Above, the schist and then the eastern gneiss succeed the 
first mentioned rocks. As will be seen in Profile viii the rocks have a 
moderate easterly dip with few variations. 

The next exposure is on the north fork of Tophet creek, where this 
series begins a few feet below the lowest outcrop of the schist, and forms 
a continuation of the canyon of the creek for over half a mile; the rock 
makes great cliff's and bluff's with a well marked strike north 10° west and 
a gentle dip of 10° to 15° east. Rock one hundred and fifty feet thick can 
be seen; the south fork of Tophet creek shows the same; here the rocks 
are much more quartzose — often a massive quartzite — and the dips are 
irregular, in some cases northerly. 

Just below the junction of the two forks of Tophet creek the water 
flows around the north end of an elliptical hill (Burlingames hill), the crest 
of which is formed by a large outcrop of massive vitreous quartzite which 
strikes north 10° east, dips 25° east. At the north end of the hill the creek 
exposes outcrops of rock with the same strike, and an easterly dip of 15°, in 
which a lenticular mass of massive quartzite passes into a dark feldspathic 
biotite schist resembling the transitions between albite schist and gneiss. 
The quartzite passes laterally as well as vertically into the schist, showing 
the sudden transitions of which these rocks are capable. We have a broad 
drift- covered area extending 1 J miles from the outcrops of massive quartzite 
on this hill to the limestone outcrops, and south to the schist in Cheshire; 
an area which contains no outcrops whatever. From the south fork of 
Tophet creek we get no outcrops of this band of gneiss until we get to 
the "point" of the mountain. This locality is a large "canoe;" that is, 
the strata turn suddenly from a north and south strike and easterly dip to 
an east and west strike with northerly dip. We have described the schist 
band and the manner in which it is overlain and underlain by white gneiss. 
The underlying white gneisses corresponding to this western band occur in 
great cliff's with a strike north 40° west and dip 15° to 20° north. From 



86 GREEN MOUNTAINS IN MASSACHUSETTS. 

their base to the base of the schist they correspond to a thickness of 450 
feet but on the theory of duphcation to only half that amount, having the 
fine-grained banded character of this western area of gneisses. These cliffs 
strike along east with the same strike and dip. The profile of Hoosac 
mountain seen from a distance shows plainly the step-like series of ter- 
races, sloping gently northward, which correspond to these beds of gneiss. 
(See PI. V, Profile xi.) Following this band of white gneiss east, at about 
one-half mile from the point of the mountain the strike has turned to north 
75° east. One-quarter mile farther there are again cliffs of this rock strik- 
ing nearly east and west and dipping nortli; the schist overlies here again. 
Beyond this point it is no longer possible to separate this band of gneiss 
from that band nearest the gi-anitoid gneiss; they merge together, after the 
band of schist has thinned out, in the great area of contorted white gneiss 
in the southern part of the field. 

NORTHERN AND EASTERN SCHIST AREA. 

It will be seen that the whole northern third of the region, and a broad 
strip along the east, is occupied by the albite schist, with commonly an 
easterly dip and north to south strike. It will be noticed that there are 
changes in the dip to the north ; on the line of the axis of the mountain the 
dip is north, but there is in general great uniformity, as there is in the case 
of this rock in the tunnel. Of course this steady dip does not mean a true 
monocline, but rather a series of folds overthrown to the west and eroded. 
No attempt has been made in the field to unravel the more minute details 
of this structure ; this was done only in important places, where the relations 
of the other rocks require it. It is also possible that troughs of the over- 
lying Rowe schist occur in this northern area, but the facts have not been 
definitely ascertained. The quartz lenses and layers, so abundant in the 
schist, are found to be always parallel to the bedding at contacts with other 
rocks of the series, where the alternation of material shows which is the 
plane of stratification, and hence these lenses can be provisionally accepted 
as indications of stratification elsewhere, when, as is often the case, the rock 
has a marked transverse cleavage.' In the vicinity of Spruce hill the schist 

'On the Greylock side cleavage lamination and stratification in the schists hare been carefully 
distinguished by Mr. Dale. 



HOOSAC MOUNTAIN. 87 

continues for some distance to have its northerly pitch, but small folds begin 
to come in, as for instance in Profile in, PI. v, parallel to the tunnel line, on 
the west summit of the mountain, where a small syncline exists. Note in 
this profile on the west slope of the mountain how the dips roll from east to 
west with commonly a northerly pitch. It is characteristic of this rock 
that it forms gorges and waterfalls along the side of the mountain. Hoosac 
mountain presents an unbroken wall for 12 miles in Massachusetts, extend- 
ing into Vermont. Profile i, PI. v, gives one of the best sections through 
the schist; it extends from the valley to the summit of Hoosac mountain and 
shows the structure here by an almost continuous section. On the slope of 
the mountain proper, the rocks have a gentle easterly dip, while at the base 
there is considerable rolling. On top of the mountain there is again a gentle 
rolling of the rocks. 

The west end of Profile i is separated by a shallow, drift-covered 
depression a few hundred yards wide from a long north and south ridge in 
the valley (see map) on the summit and sides of which we find the typical 
Hoosac albite schist, often very garnetiferous, extending in an almost straight 
line to near the western portal of the tunnel, where it stops. This ridge of 
schist is everywhere separated from that of Hoosac mountain by this small, 
drift-covered hollow, so that we have only the lithological identity to cor- 
relate by. This rock is succeeded by the limestone on the west tlu-oughout 
its extent. Profile ii, PI. v, shows the relations of the rocks across this 
ridge, beginning with those which are exposed on the north fork of the 
Hoosic river in North Adams. The Stockbridge limestone has here its most 
northern outcrop in Hoosic valley and strikes north 20° east, the dip varies 
considerably; the rock is much folded, a fact well shown in a quarrv and 
chasm in the limestone at the "Natural Bridge." This rock is succeeded 
within 60 feet by a schist with conformable strike, and dip east 40°. About 
800 feet across the strike east from this contact, with one or two intervening 
outcrops of schist, we have a high bluff along the river, composed of mica- 
ceous schistose limestone, effervescing strongly with acid, striking north 25° 
east and dipping 25° east. This bluff extends for some distance and is 70 
feet high, exposing a considerable thickness of the rock. At the top of the 
bluff there is a flat bench, gently rising to the east (evidently formed by 



88 GREEN MOUNTAINS IN MASSACHUSETTS. 

this rock) for nearly 500 feet, then rising more steeply to the summit of 
this ridge, where we find the albite schist with the same strike, but greatly 
cnimpled dip. There are no outcrops between the top of this bluff of lime- 
stone and the schist, about 3,000 feet horizontally. No outcrops are found 
for a mile south of this place along the strike, then we find the limestone in 
a small quaiTy, striking north 35° east, dip 35° east. This limestone at the 
top of the quarry is conformably overlaid by a black schist, and 50 feet dis- 
tant across the strike an outcrop of the typical Hoosac schist has the same 
strike, crumpled in small folds with a northerly pitch. It looks very much 
like a transition from limestone to schist at these places. From here there 
are few outcrops down to the West Portal, where the schist entnely runs 
out just north of the tunnel. There seems to be in this ridge a trough of 
schist with a pretty steady north to south strike and crumpled dip. The 
outcrops can be traced along the summit of the ridge almost continuously. 
The northern area of schist overlying the Vermont conglomerate south 
of Spruce hill soon turns from the east and west strike as we go east to the 
steady north and south strike of the eastern border, and runs from here with 
an almost straight line to the southern border of the sheet. The conformable 
contact with the white gneiss (Vermont) at the pond (Profile v, PI. v) 
has already been mentioned; the line of contact runs about 9 miles to a 
point about 1 mile northeast of Windsor Hill, where the contact is well 
shown between the fine grained white gneiss and the schist; strike north 
20° east, dip steep east. _ There are here transitional beds between the 
gneiss and schist formed by very micaceous layers. Over a mile due east 
of Windsor Hill the same thing occurs again; the schists are here very 
garnetiferous. The Rowe schists, which lie east of and hence overlie the 
Hoosac (albite) schist, have been mentioned previously. They . appear 
on the map (PI. i) as a narrow strip at the eastern edge, passing into the 
Hoosac schist at the line of contact. They will be described in their more 
general relations in a forthcoming memoir of Prof. B. K. Emerson covering 
the territory east of the map. 

THE REGION SOUTH OF CHESHIRE AND OF THE HOOSIC VALLEY. 

The area of gneisses (Vermont formation) south and southeast of the 
granitoid gneiss can best be described by beginning at the southwest end. 



HOOSAC MOUNTAIN. 89 

In the Hoosic valley here we have the Stockbridge limestone crossmg 
the valley from the Greylock side and running close up to the slope of the 
hills on the east side. This limestone is succeeded by a broad band of 
quartzite (Vermont) on the slopes of the hills and this again by a series of 
gneisses (Vermont) which extend to the crest and back from it, east. In 
the southwest part of the map the quartzite occurs in a long ridge running 
northerly and southerly, just east of the Hoosic river. It is a very massive 
vitreous variety, the dip of which is obscure. A little hollow, perhaps a hun- 
dred feet wide, separates it from the gneisses on the east, which strike north 
25° east parallel to the trend of the quartzite, and dip first west then east — 
much folded. Following 1 mile north from here without finding out- 
crops, we come to a creek running into the large pond in the valley a few 
hundred yards north of Berkshire depot. Just where this creek issues from 
the sloping benches a little east of the road we find well-marked ledges of 
the limestone striking north 37° east, dip steep westerly; 125 feet east the 
next outcrop dips east 65° and is in contact conformably with a calcareous 
quartzite; for one-half mile or more up this creek beds of this calcareous 
quartzite are found, in places massive quartzite; then, after a covered inter- 
val of 400 feet, we find ledges of laminated gneiss (quartzose) dipping also 
east 50° (strike north 40° east); farther up the creek this gneiss is succeeded 
by coarse gneisses with blue quartz resembling the granitoid, also dipping 
east. We have here a transition of the Stockbridge limestone into the Ver- 
mont quartzite, and this is in turn overlain by gneisses, the whole series 
inverted. The limestone is covered along the contact from here north to 
Cheshire. The line of contact between quartzite and gneiss can be easily 
followed north along the side of the mountain, the two rocks never quite 
in contact, until we reach a point on the side of the mountain half a 
mile south of the north end of the pond; here the quartzite and underly- 
ing fine grained gneiss make a sharp turn, and, as is so often the case in 
this region, in the turn the rocks are not eroded away. The southernmost 
outcrop of a laminated quartzite strikes north 45° east, dips 60° west; across 
a littl-e ravine to the north this curves to strike east and west, dip 50° north- 
erly. It is overlain by a large bed of very massive vitreous quartzite, and 
near the outcrops of the latter numerous angular blocks of a quartzite-brec- 
cia cemented by limonite occui- — a rock often found in these sharp turns in 



90 GREEN MOUNTAINS IN MASSACHUSETTS. 

the quartzite and connected witli the crashing. The laminated quartzite is 
closely underlain by curving outcrops of a rather coarse layer gneiss, in 
which long flat bands of feldspathic material, blue quartz, and biotite 
alternate. This again is conformably underlain by outcrops of fine-grained 
biotite gneiss. These outcrops are separated a few feet horizontally. 
Their contacts must be within a few inches of strata, and tliey are perfectly 
conformable. This proves the structural conformity of this massive quartzite 
series with the underlying gneisses. A mile and a half north of this we 
find the sharp point of the mountain, on the east side of which the valley 
makes a bay or "cove" running a mile south. This "point" of the moun- 
tain is formed by the massive quartzite, south to the crest, and also at its 
north and west base, where the quartzite is quarried for sand, and the stream 
makes a fine cut through it. One-eighth mile east of Cheshire village the 
quartzite is quarried from a large mass, striking north 30° west, dipping 20° 
northerly, and can be followed southeast for at least one-quarter of a mile 
with the same strike and dip. Along the west side of this point of the 
mountain the quartzite has been quarried in several places. About 1 mile 
south of Cheshire, near the north end of the pond, at a sand mine, the 
quartzite strikes north 40° to 50° east, dips 20° west, while northeast of here, 
on the slopes of the mountain, near another old sand-mine, the strike is 
north 80° west, dip 20° northerly. Tliis "point" of the mountain therefore 
represents an anticline in the quartzite, collapsed and overthrown to the 
east — a prow, < »r inverted canoe. On the top of the crest of the mountain 
the quartzite forms the slopes and highest crests, striking north 15° west, 
dip 15° east; in the east slopes it strikes north 30° west, dips 30° east. 

Going back to the quartzite quarry, in a little ravine off" the road, an 
outcrop of calcareous quartzite is found overlain within 10 feet by an im- 
pure limestone. The strike is about north 20° west, dip about 30° north- 
east. A few hundred yards further north outcrops of limestone are found 
striking north 50° west, dipping 45° east. It is to be noticed therefore that 
the limestone also circles around the quartzite to the north and strikes south 
to lie east of the quartzite, forming in part at least the bay or "cove" of the 
valley. No outcrop, however, of the limestone in place is found in this 
cove. The southern rim of the cove is formed by massive quartzite which 



HOOSAC MOUNTAIN. 91 

strikes north 85° east, dip 50° northerly, gradually turning on both sides 
of the cove tp a north and south strike. Thus on the east side of the cove 
it strikes north 65° east and dips west; approaching the succeeding point 
of the mountain it strikes north and south, then at the extreme of this point 
north 37° west, dij) vertical. The extreme point is formed by a very massive 
vitreous quartzite, 150 yards nortli of which there is a loose outcrop of lime- 
stone, probably not in place. There are also small ledges of schist on the 
west edge of the cove which probably are in place; strike north 32° east, 
dip west steep. They show that the schist area north of the cove runs in 
here near the quartzite. As we go east from this second point the quartzite 
strikes north 30° west, dip northeast, then begins to strike east and west 
and dip northerly with a constant strike. About a mile from this second 
"point," or sharp canoe, in the quartzite, we come to a very important local- 
ity, where this massive quartzite and conglomerate passes along the strike 
into the white gneiss series of Hoosac mountain. Half a mile from the 
second "point" the massive quartzite runs up the hill, striking north 80° 
east, dip northerly 80°. A great thickness of massive quartzite is exposed 
here ; in some cases there are beds of well-marked conglomerate with quartz 
})ebbles; this quartzite runs in great cliffs up the side of the mountain (see 
map, PI. i). As it approaches the summit it becomes more and more 
micaceous. At the summit and near the north to south road running to 
Windsor, it changes along the strike within 200 feet into a fine-grained white 
gneiss. The quartzite on this hill is separated into two divisions by a layer 
of black biotite schist of some thickness Tlie rocks turn around this hill, 
which represents a quartzite dome (the rocks dipping north), and then by 
their dip are carried down to DrA' brook, to which they can be easily traced 
by long cliffs and scattering outcrops. 

This brings us to the area between Dry brook on the south, the "point 
of the mountain" north (where the central series of Hoosac mountain 
makes its sliarp turn to the east), and the western border of the Hoosac 
schists on the east. The rocks we find in this area are varieties of the 
white gneiss, often coarse. Along tlie western border there are quartzites 
and conglomerates interbanded with gneisses, wliile the large area of schist 
in Hoosic valley extends east into the gneiss area. Three general pecu- 
liarities of structure may be noted (see map, PI. i) : 



92 GREEN MOUNTAINS IN MASSACHUSETTS. 

First. Ill the west part of tlie area, between Dry brook and the curve 
north (about 2 miles from north to south and 1 mile wide), there is a quite 
steady strike about north 50° west and moderate northerly dip; a perfect 
monoclinal structure. 

Second. In the belt east of this, 1 mile or more wide (on the map the 
central area of flat summits), the gneisses are greatly curved and twisted. 

Third. In the belt extending from the previous one to the border of 
the schists the normal north to south strike occurs with predominating east- 
ern dips, as in the schists. 

This east and west strike and monoclinal north dip was a matter diffi- 
cult of explanation, as there appeared to be a great series of gneisses and 
quartzites, thousands of feet in thickness, underlying the series of the north- 
ern part of Hoosac mountain. It was not until the white gneiss- conglom- 
erate and schist tongue had been traced around the core of the gi'anitoid 
gneiss, and it had become evident that there was an underturn of these 
rocks, and that they were really geologically above the granitoid gneiss, as 
in their normal position in the region of the tunnel, that it was possible to 
exjjlain the monoclinal dip of the gneisses further south. It is now believed 
that this is due to a series of east-west transverse crinkles, pushed under 
and collapsed from the south, so that there is a constant duplication of 
strata in an apparent conformable series. One proof of this theory is the 
fact that we find the actual connection between two adjacent layers of the 
monoclinal series in several cases on the west brow of the mountain. 

Ill one case a band of the gneiss having the schist both north and south 
of it was traced continuously along the strike for a half mile. It gradually 
turned to a northerly direction, the schist closely following, and then came 
to an end, the gneiss terminating in a small crumpled outcrop and the schist 
each side circling around and joining. The zone nearer the schist on the 
east, with general north and south strike and easterly di]), must represent a 
large series of similar north and south folds overturned to the west, and 
the areas of extremely crumpled gneiss between the two represent the 
turning point where the east and west folds ai'e twisted around to the north 
and south direction. 

In the following details the reader should refer to the map (PI. i), on 



HOOSAC MOUNTAIN. 93 

which the observations are platted. In the previous descriptions the Ver- 
mont quartzite had been followed to where the lower part passed into schist- 
ose quartzite and finally into banded white gneiss, and had been traced 
down to Dry brook. The upper layer of quartzite also is carried down to 
Dry brook and appears in massive ledges along the brook, just where it 
issues from the mountain. It is quarried here in a sand mine and runs up 
the brook several hundi-ed feet in great ledges, striking north 35° west, dip 
northeast 25°. In one place, a few feet west of the sand mine, the quartzite 
forms an iron breccia, which is evidence of crushing. From the sand quarry 
this quartzite can be traced along the strike for a quarter of a mile into 
the region of the gneiss. At first it forms a massive quartzite in bluffs ; 
then bands of micaceous gneiss come in; and there are alternating layers a 
foot or two wide of pure quartzite and layers of finely banded white gneiss. 
These changes are well shown in this distance. The transition from quartz- 
ite to gneiss is unmistakable and plainly to be followed. There are ledges 
of rock here which have elongated pebbles resembling the conglomerate. 
For a mile north we have a series of fine-grained, banded white gneisses, 
with steady strike north 40° to 50° west and northerly dip, which on the 
west slopes of the mountain towards the valley are greatly contorted, the 
layers of the monocline doubling on themselves and running back in a 
manner which it would be impossible to describe in detail. 

At a point a mile north of Dry brook, just on the west edge of the 
mountain, we find a large blufi" of gneissoid conglomerate, the flattened 
pebbles composed of quartz grains, while muscovite and biotite plates and 
some feldspar, with octahedra of magnetite form the cement — a gneiss. 
The rock is often banded, bands of mica-schist alternating with those of 
conglomerate. The ledge strikes north 40° west and dips 40° northerly. 
The continuation of this series of rocks can be traced over a mile southeast 
with about the same strike and dip. This bluff" is on the west crest of the 
mountain. When we go north from this outcrop we can trace this series 
of conglomerates within a space of about a quarter of a mile to outcrops 
with northeast strike and steep northerly dip, then east and west strike with 
northerly dip, and then the same oi'iginal strike north 40° west, dip north- 
east, with which we started; the rock then strikes southeast into the gneiss 



94 GREEN MOUNTAINS IN MASSACHUSETTS. 

area of the Hoosac mountain, where its character is lost. Thus we have 
here the case of two layers of the monoclinal series joining to form one 
double band, the connection made by a series of curving layers at the west 
edge of the mountain. This conglomerate is bounded on the west by beds 
of massive quartzite which can be traced by loose pieces along the moun- 
tain side nearly to Dry brook, where they connect with the quartzite of the 
sand quarry. By what complicated crumpling this is effected it is difficult 
to say. 

In the httle brook running west down the side of the mountain, about 
midway between Dry brook and the turn of the mountain, we have an 
important contact between the schist (forming the large area in the valley) 
and the (Vermont) quartzite of the side of the mountain. The two rocks 
are conformable, strike north 35° west, dip 30° northeast. This schist 
extends north to the turn of the mountain, there running in east among the 
gneisses for some distance; it is impossible to describe the contoiiion it has 
undergone; it is in general a series of small minor folds whose axes dip 
northerly with the dip of the strata. The line of outcrop is hence very 
winding and iiregular. In places just here the schist assumes the form of a 
massive iron schist composed of quartz grains, magnetite, graphite, and 
biotite, which is easily followed. About half a mile south of the turn it will 
be noticed on the map (PI. i) that the gneiss (^'ermont) sends a cur^^ng 
tongue northward surrounded by schist on either side; we have in this 
another good proof of the real duplication of layers which causes the mono- 
clinal dip of the gneisses. The schist and gneiss are conformable and follow 
each other closely to the point where curATng layers of schist circle around 
the gneiss and cut it off. ■ It is a very sharp anticlinal curve, the gneiss 
doubling back on itself wnth the schist closely following. (See p. 92.) 

In a small brook flowing west at the point of the mountain, just below 
the cross roads we find again the schist in conformable contact with a 
quartzite which here overlies it. Both sti'ike north 45° east and dip west 
gently. A few hundred feet east a quartzite white gneiss is found overly- 
ing the black modification of the schist mentioned above, which can be 
traced along in bluffs for nearly a mile, forming the base of the western 
band of white gneiss, where it has tmnied to mn east. About a mile dis- 



HOOSAO MOUNTAIN. 95 

tant it forms locally a crumbly quartzite which has been quarried; in the 
intervening- space we have the same phenomenon of transition of quartzite 
to gneiss described before near Dry brook; that is, we have small layers 
or lenses of the quai-tzite in the gneiss. 

West of the contact of schist and quartzite under the bi'idge, the two 
rocks extend some hundred feet downstream; then they rise together to 
the bluff's and run into the open meadows, where we find outcrops of biotite- 
gneiss overlying the quartzite. No contact with the quartzite can be found, 
but the three rocks follow one another in several sharp turns, in which they 
seem to conform in structure. The strike turns within 300 yards from 
north 60*^ west, with northeast dip, to north 45° east, with westerly dip. 
This can-ies the rock down southeast to an outcrop along the road, where 
we have in place a large ledge of the quartzite-breccia indicating a sharp 
turn. Some hundred feet northeast an outcrop of the quartzite strikes 
north and south, dipping east. These outcrops are scattering, and from 
this point north we have a large drift-covered area with no outcrop what- 
ever (see map, PI. i). They are mentioned in detail because they occur 
in the south end of the hill in which "Burlingames" massive quartzite is 
found, about half a mile distant, and it seems probable that this is the same 
quartzite very much crurnpled (corresponding to the "canoe" in which all 
the rocks here are folded). This enables us to connect it with "Burlin- 
game's" quartzite and with the line of quartzite observed at intervals all the 
way south from the tunnel line. 

We have heretofore been dealing with the boundaries of the great area 
of (Vermont) gneisses and quartzites between the Stockbridge limestone on 
the west, the Hoosac schists on the east, and the granitoid gneiss (Stamford 
gneiss) on the north, covering on the map parts of Windsor, Dalton, and 
Savoy. The attempt has not been made to determine in detail the structure 
of the interior of this mass, although a glance at the numerous observations 
on the map will show that the ground has been fairly well covered. It is 
impossible, so far as our work has gone, to recognize definite horizons within 
this mass, and without these it would be hopeless to trace out the exact 
structural features. 

It was mentioned, in speaking of the contact of the Vermont quartzite 



96 GREEN MOUlvlTAlNS IN MASSACHUSETTS. 

and Stockbridg-e limestone, that the quartzite was succeeded by gneisses 
with conformable strike and easterly dip, which are often quite coarse, 
with blue quartz, resembling the granitoid gneiss. This feature can be 
noticed at several places; for instance, east of the exposure of quartzite 
at the extreme south end of the map. We go east for nearly a mile, find- 
ing gneisses, part coarse, part fine, and then come to massive quartzite, 
and well-marked conglomerate (not metamorphic gneiss-conglomerate), with 
pebbles of blue, white, and black quartz. The quartzite also circles around 
the eastern part of this area in Dalton (south of the limits of this map), 
where it is again associated with limestone. We find rather contorted 
gneisses in the central part of this area, under the word "Dalton" on the 
map, and farther north massive quartzite with north and south strike and 
varying dip, which is the southern continuation of that forming the sharp 
quartzite "points" of the mountain in Cheshire. So this part is evidentl}" 
composed of numerous north to south troughs of the quartzite and conglom- 
erate, with areas of the underlying gneiss, the quartzite covering the gneiss 
at both ends and being folded under it on the west. 

This statement is also true of an area running south from the second 
point of the mountain, where the rocks are quartzite, quai'tz-schist, and 
quartzose gneisses, with beds of quartzite-conglomerate, the strike being 
north and south and dip steadily east. 

In the region directly south of Dry brook we have coarse gneisses with 
blue quartz, underlying the fine grained quartzose gneisses (Vermont) which 
represent the quartzites, and therefore perhaps correspond to the granitoid 
gneiss (Stamford gneiss) of the central part of Hoosac mountain. 

In Windsor we have the same series of white gneisses, the conglomerate 
character not marked, it being probably too far east, and the increasing 
metamorphism having perhaps masked the original characters. 

A large part of this area is very poor in outcrops, being flat and drift- 
covered. We have therefore described this large region principally in 
reference to its boimdaries, where by the contact with other rocks the true 
relations and structure can be determined, and we hope that our observations 
establish — first, the conformity of the Stockbridge limestone and Vermont 
quartzite, the latter underlying when in the normal position, as is shown by 
the contacts and lithological passage and the fact that the limestone is sharply 



HOOSAO MOUNTAIN. 97 

folded with the quartzite; second, the identity of the quartzite-conglom- 
erate horizon underlying the limestone (that is, the Vermont quartzite) with 
the fine grained white gneisses of the Dalton- Windsor area, and of these 
with the white gneiss series of the central mass of Hoosac mountain ; third, 
the conformable contacts of the schist area in Hoosic valley with members 
of the quartzite-white-gneiss series. 

HOOSIC VALLEY SCHIST. 

We have still to take up the relations of this large schist area to the 
limestone. This rock is a typical schist, often garnetiferous, coming in places 
close to the quartzite — at the "cove" within 250 yards. Near the quartzite 
tongue on the western side of the "cove" we find the ground filled with 
loose pieces of limestone and schist, with beautiful transitions between 
the two rocks caused by the presence of the twinned plagioclases of 
the schist in the limestone (see p. 64). It may be mentioned that the 
same rocks occur in the beds of Mount Greylock. Only loose pieces of 
this transitional material occur here, with one exception, but as they are 
nearly on the line of contact of limestone and schist it can fairly be pre- 
sumed that they are nearly in place and represent direct contact; one ledge 
alone is exactly in place. The contacts of this schist with the quartzites of 
the white gneiss series have been mentioned; in one case the schist under- 
lies, in the other overlies. In the former case, near the large "canoe," we 
know that the white gneiss series is inverted; in the other we know that it 
must be normal, and hence the position of the schist as overlying the 
quartzite-gneiss is made clear. The Stockbridge limestone bounds this schist 
on the west and northwest. At the southwest corner no contact is found, 
although the two rocks come quite close together, the schist forming a hill, 
the limestone lying in the valley at its base. The contact (concealed) runs 
along to Cheshire Harbor, where limestone and schist are within 20 feet 
horizontally. The two rocks have the same strike, north 35° east. The 
dip of the limestone is 30° westerly; that of the schist is obscure, but 
appears to be westerly. This seems, therefore, to be a conformable juxta- 
position, although actual contact is wanting. The line of contact runs 
north for a mile, then doubles around the north ridge of the schist and runs 

MON XXIII 7 



98 GEEBN MOUNTAINS IN MASSACHUSETTS. 

southeast. Where it crosses Dry brook we fiud massive hmestone within 
a few feet of the schist, and the hmestone seems to dip under the schist. 
There is also exposed in the brook, near the contact, interbanded hme- 
stone and schist near the contact of both rocks, just as observed in North 
Adams (see p. 88). The hne of contact just here is very irregular, zigzag- 
ging, as we should expect in these crumpled, sharply folded rocks. At 
the south end of the lenticular hill north of Dry brook the outcrops disap- 
pear for over a mile, when we come to Tophet brook, where we have the 
gneiss, quartzite, and limestone in close contact, as previously described. 
From here north to the locality in North Adams descril^ed (p. 88) the 
contact of the limestone is concealed on the east, although in places very 
close. The structure is given on the map (PI. i and iv) by strikes 
and dips. North of the North Adams locality no limestone in place has 
been discovered. The head of the valley containing the north fork of the 
Hoosic river, some 8 or 9 miles from North Adams, is formed by the 
schists of the northern part of Hoosac mountain. The limestone evidently 
runs up for some distance from North Adams, covered with drift, and then 
disappears. 

THE REGION AROUND CLARKSBURG MOUNTAIN AND STAMFORD, VERMONT. 

This brings us to the last area to be described in this report, namely, 
the mass of Clarksburg jnountain, northeast of Williamstown and northwest 
of North Adams. As will be seen by the map, the north and south forks 
of Hoosic river unite at North Adams and flow due west through an east to 
west valley, lying between the north end of the Greylock mass and the 
south slopes of a high mountain mass extending down from Stamford, Ver- 
mont, into the town of Clarksburg, Massachusetts. 

We find the Stockbridge limestone in the streets of North Adams (see 
map, PI. i) and in the high ridge just south of the railroad, where it is found 
in contact with and overlying the Mount Greylock Berkshire schist. The 
latter rock is cut through by a raih'oad tunnel just west of the North Adams 
depot, where the limestone forms part of the eastern side of the Greylock 
synclinorium, really underlying the Berkshire schist, but here inverted by 
a sharp, overturned fold. 



HOOSAC MOUNTAIN. 99 

The summit of Clarksburg- mountain is composed of a mass of granitoid 
gneiss (Stamford gneiss) identical in petrographic characters with that of 
the Hoosac tunnel (Stamford granite). This is overlain by the Clarksburg 
quartzite (Vermont formation) on the west and south sides, and by quartzites 
and gneisses on the east side, the contacts having been found. In this 
quartzite Mr. Walcott has found the remains of trilobites, making it Lower 
Cambrian, and we sliall now endeavor to show thnt this is represented by 
the gneiss found on the east side of the mountain. 

Near the old signal station on Clarksburg mountain the quartzite is 
represented at the immediate contact by a blue quartz pebble conglomerate, 
quite micaceous, the pebbles composed of aggregate quartz. Some distance 
ab(3ve the contact the quartzite contains beds of a quartz schist of consid- 
erable thickness. The quartzite and conglomerate are found within 2 or 3 
feet of strata of each other, the quartzite striking on the average about north 
33° west, and dipping 25° southwest. The granitoid gneiss in part has 
little structure, but in several places this feature is well marked by the mica 
planes, which are in general parallel both in strike and dip to those of the 
quai-tzite, so that in so far as we can accept as stratification such structui-al 
planes in the gneiss, the two rocks are parallel. From this place, on the 
northwest edge of the mountain, the line of contact, curving gently, runs to 
the southeast brow of the mountain above North Adams, where it turns 
and strikes northeast. The contact here between the two rocks is very 
close, and the stmcture of the granitoid gneiss obscure. The rock is massive. 
The quartzite strikes north 30° east, dips 40° southeast. The line of contact 
across the mountain can be traced in a general ^^''ay, but no outcrops near 
together have been found. 

The whole south slope of the mountain down to the valley is covered 
with the quartzite and the intei'banded quartz schist. The southwest dip is 
well marked above Williamstown, while on the North Adams side it is south- 
east. This mountain is a large quartzite dome, doubtless with many minor 
crumples. This quartzite is found as low down as the river bank opposite 
the cemetery in North Adams. It is last seen in contact with the granitoid 
gneiss at the place mentioned above, but it is thence eroded away to the 
north for a distance of 2| miles, in which drift covers the valley and lower 
slopes of the mountain, the granitoid gneiss occupying the crest. 



100 



GEEEN MOUNTAINS IN MASSACHUSETTS. 



Just north of the Massachusetts state line, in Vermont, about 2 J miles 
northeast of the last contact, we find again the contact of the granitoid gneiss 
with quartzite; this is in Stamford, in the liills west of the village.^ 

The gi-anitoid gneiss has the same general characters that it has further 
south. The contact is found near an old schoolhouse along the roadside. 
The quartzite is micaceous and strikes north 30° to 55° east, bemg curved 
a little in the outcrop and dipping 42° east; the contact is seen here within 




Fig. 28.— Contact of granitoid gueias (Stamford gneiss) and quartzite (Vermont formation), Stamford, Vt. Looking 
nortli. 

Tlie gneisa tills the left half of the figure. It is here very coarse, with structure feebly indicated. The hollow in 
its center (through which the road goes) is caused by the erosion of a vertical dike of amphibolite about 11 feet wide, 
which does not penetrate the (xuartzite. The quartzite is seen on the right, dipping southeast. 

1 foot of strata, and by digging the actual contact was found. The lamina- 
tion of the granitoid gneiss strikes north 55° east, dips about 40° easterly; 
that is, in a general way conformable to the Ijedding of the quartzite. At 
this place a vertical band of rock 14 feet wide strikes north 60° west, or 
across the strike of l^oth rocks; it has the character of the altered rocks 
described on pages 65 to 69 and is undoubtedly a dike; this runs in a 
straight line through the granitoid gneiss, but abuts against the quartzite 



' C. H. Hitchcock briefly describea this locality in Geology of Vermont, p. 601. 



HOOSAC MOUNTAIN 



101 



without passing into it, and the quartzite has a curious thickening of its 
layers where the dike joins it, as though there had been a hoUow, owing to 
erosion of the dike before deposition of quartzite. It seems therefore to 
show the most perfect unconformity between the granitoid gneiss and the 
overlying quartzite, although the lines of structure of both rocks are parallel. 
(See Figs. 28 and 29.) We can trace this contact northward for a quarter of 
a mile or more; the quartzite is interbanded with very feldspathic gneisses, 
the whole forming quite a thick series. The rocks dip east (43° east, strike 




Fig. 29. — Contact of granitoid gneiss and quartzite; same locality as 28, looking east, showing the quartzite nearer. 
Tlie dike was foi^nd, by digging, to lie against the quartzite without passing into it, and the quartzite shows a curi- 
ous lenticular thickening just in the line of the dike, as though there had been a depression there at the time of deposit. 

north 40° east) and so does the structure of the granitoid gneiss. Between 
this point and the quartzite above Noi'th Adams one outcrop of quartzite 
conglomerate has been found in place, strike north 45° east, dip 30° east. 
There seems therefore no doubt that this series of quartzites and gneisses, 
lying on the granitoid gneiss without a fault, are the same as the quartzite 
at North Adams, 2 miles off: they have the same strike and dip and lie 
on the same rock, and a glance at the map will show that the line of strike 
runs from one to the other. We have here then the second proof that the 



102 GREEN MOUNTAINS IN MASSACHUSETTS. 

white gneiss-cong-lomerate of Hoosac mountain is the Cambrian quartzite 
(Vermont). 

GENERAL CONCLUSIONS. 

In the previous pages a presentation of the facts observed has been 
attempted without drawing conclusions or stating results. A brief sum- 
mary is therefore hei'e introduced. 

The rocks of Hoosac niountain consist of quartzites, conglomerates, 
gneisses, limestones, schists, and amphibolites. In all these rocks there is 
abundant evidence that some elements have been crushed b}- great pres- 
sure; the large broken microcline and quartz masses of tlie coarse gneiss 
and the pebbles of the conglomerate show this, and this crushing has 
been accompanied by chemical action which has formed new feldspar, mica, 
and quartz. With the exception of the pebbles of the conglomerates, it is 
with great difficulty that we recognize the remains of detrital material, and 
yet a large part of the series is of detrital origin. The rocks as we now 
find them are thoroughly metamorphic, and yet we feel sure, that the 
material for the present rocks must have come from the old sediments. To 
trace the process of change is a problem of the future. If, as tliis work indi- 
cates, these rocks are simply the Cambrian and Silurian sandstones, lime- 
stones, and shales, altered by a metamorphism inci'easing from the Hudson 
river eastward, then careful petrographic studies along an east to west 
line ought to solve this problem. A partial investigation of some of the 
rocks of Mount Greylock, made by the writer, shows the great similarity 
between the metamorphic rocks of Hoosac mountain and of Grreylock, 
qualitatively considered, but in quantity jhe difference is striking. There 
are no coarse gneisses on Greylock, and it is only locally that fine-grained 
banded gneisses are found, but limestones, quartzites, and schists (or phyl- 
lites) abound, and we must again state the absolute lithologic identity of 
these varieties with those of Hoosac. The schists of Mount Greylock and of 
the Taconic range have the same crystals of albite and the same ottrelite ; the 
limestone of Greylock is feldspathic, just like that at the base of Hoosac. It 
is then a suggestion worth considering whether the metamorphism does not 
increase as we go downward as Avell as eastward. The schists of Greylock 
and those of Hoosac at the top of the series are alike; the coarse gneisses 



HOOSAC MOUNTAIN. 103 

at the base of the Hoosac series are not found in Mount Greylock or in 
the Taconic range, at least not here. I am not pre})ared to say that the gran- 
itoid gneiss itself might not be an altered sediment, instead of an eruptive 
granite affected by dynamic metamorphisra, but in such an extreme case 
we need careful proof of the process of change, which we can not yet give. 
This rock has perhaps rightly been called Archean by J. D. Dana, C. H. 
Hitchcock, C. D. Walcott, and others, the proof resting on some litho- 
logical resemblance or on unconformity with the overlying rock. It has 
been shown in the previous pages that this evidence is unsatisfactory, for 
the most absolute conformity exists in places, and the overlying rocks some- 
times take on the characters of the granitoid gneiss. The altered trap dike 
found in Stamford, which cuts the granitoid gneiss but not the quartzite, is 
the first conclusive evidence of nonconformity. 

Another striking fact is the uniform result produced by metamor- 
phism in the originally dissimilar rocks. The amphibolites were primarily 
trap rocks composed of hornblende and feldspar, and even the hornblende 
may have been derived from augite and the rock a diabase ; but this fact, 
proved for rocks in other regions, is yet in doubt here. By the metamor- 
phism of these eruptive rocks new feldspar, biotite, hornblende, etc., are 
formed — of which minerals some occur with the same peculiar features 
(feldspar) in the schists which have been formed from sediments (shales, 
slates, etc.). In the process of metamorphism here there must have been 
an important chemical action originating from without the rocks. 

A further unexplained condition i^ the vertical position of the plane 
of lithologic change toward a gneissic character. The fossiliferous Cam- 
brian quartzite (Vermont) of Clarksburg mountain forms a great dome, on 
the east side of which it strikes northeast toward the crystalline rocks, and 
within 2 miles, in Stamford, Vt., we find it partially changed to gneisses. 
The quartzite of Cheshire preserves its character as quartzite until its strike 
carries it east across a certain meridian (the west crest of Hoosac moun- 
tain), then in a quarter of a mile, passing this line, it gradually changes 
into a white gneiss by taking up feldspar and mica. A mile or so nortli we 
find that the ends of the little cross-crinkles in the white gneiss nortli of 
Dry brook are quartzite and ordinary quartzite-conglomerate. They pass 
into white gneiss when they strike east within a very short distance. 



104 GREEN MOUNTAINS IN MASSACHUSETTS. 

Lastly, there is the limestone wliicli on Greylock underlies and is inter- 
stratified with the schists; we find this In Hoosic valley close to the gneiss 
and quartzites, but no sign of it on the mountain proper. Reviewing the 
evidence bearing on the position of the limestone, we have on Hoosac moun- 
tain a conformable series — granitoid gneiss, overlain by a white-gneiss-con- 
glomerate-quartzite formation, and this by schist. We trace along the strike 
the quartzite of Hoosic valley into tlij? white gneiss-conglomerate-quartzite 
series underlying the schists; and we also trace the same Cambrian quartzite 
of Clarksburg mountain into white gneisses. This quartzite of Hoosic val- 
ley we find in several localities passing upward into the limestone; it is Prof. 
Dana's quartz rock which underlies the limestone. This quartzite we trace 
also laterally into the Hoosac mountain white gneisses, and we find the schist 
which borders the limestone of Hoosic valley in several confonnable con- 
tacts with the mountain quartzites and white gneisses with no intervening 
limestone. We find near the contact of schist and limestone perfect trans- 
itional feldspathic micaceous limestones (not all in place) and near North 
Adams very close proximity of the schist belonging to Hoosac mountain with 
limestone. There seeems to be conformity between all the rocks, and yet 
the limestone is wanting in the mountain section. The only solution would 
seem to be that the limestone is replaced by the schist on the other side of 
the line or plane mentioned above, whether it be an original shore line, or 
some bounding line or plane of certain conditions of metamorphism peculiar 
to the axis of the Green mountains. To bring in a fault or tlu'ust plane at 
the base of the Hoosac mountain, cutting off the crystalline rocks of the 
Green mountains from the fossiliferous rocks west, is an easy solution of a 
difficult problem, l)ut not the correct one if the facts are correctly inter- 
preted.^ 

There remain to summarize the facts bearing on the stratigraphy of 
Hoosac mountain. The reasons for the conclusions as to the general struc- 
ture of Hoosac mountain need not be recapitulated here ; it is an anticlinal 
fold, the axis of which lies nearly in the meridian. This axis is not horizontal, 
but inclines or "pitches" (to borrow a term used for similar folds in the New 

' The reader is referred to Part i for a further discussion of the condition of the Hoosac and Grey- 
lock columns. 



HOOSAO MOUNTAIN. 105 

Jersey iron ores) 10° to 15° to the north. It is this pitch which enables us 
to get the series of rocks in normal position and measure their thickness, 
just on the axis of the fold, for on tlie sides we could never have known 
which rock was the upper or the lower, owing to inversions, or whether the 
apparent thickness was not produced by duplication of a thin layer by 
frequent closed and overturned folds, as is the case at the southern end of 
the field. t 

This anticline preserves the rocks in their normal position on the east 
side, but on the west they are folded under in inverse position, with eastern 
dip. (See Profile v", PI. vi). It is also proved that at the south end the 
rocks have been pushed in under, so that they dip north instead of south, as 
they would naturally do if the fold terminated in another dome at its south 
end. Where the normal east side of the anticline and the underturned west 
and south sides meet we find a great crumpling, and then the two sides 
come together and the whole series strikes north to south. The long, thin 
tongue of schist which runs south from the main mass is confoi-mable to the 
gneisses on both sides of it, and must therefore lie in a naiTow trough in the 
white gneisses which terminates at the south end. The second or west band 
of gneisses, judging from its conformity to the schist and from the fact that it 
runs into the larger area of gneiss as one of the series, after the schist tongue 
ends, must be considered identical with the gneiss next to the granitoid 
gneiss, except that in this western band it has more of the quartzite and 
less of the gneiss character, corresponding to the general change across 
this meridian. This western band would in that case represent an over- 
turned anticline in the white gneiss, really overlain by the limestone, 
which by the overturn is made to dip under it. This anticlinal trough 
of white gneiss pitches under the schist north of the tunnel. Lastly, if 
the limestone and schist are the same rock we must suppose that the 
change from one to the other took place in the eroded portion of the 
arch which connected the limestone with the trough of schist. Profile 
v", PI. VI, illustrates this theory. I am well aware that such an explan- 
ation seems forced. It would be much more plausible to say that these 
formations are separated by north to south faults, but all the evidence goes 
against the existence of faults. Where formations are found to overlie each 



106 GREEN MOUNTAINS IN MASSACHUSETTS. 

other conformably at so many points and to curve around in conformity, as at 
the southwest comer of Hoosac mountain, no kind of fault could explain 
the relations. In fact, faults on a large scale seem to be absent, although 
considerable breaking may have accompanied the great crumpling. On 
the summit of the mountain east of Berkshire, near the extreme southern end 
of the map, a small fault was found between quartzite and schist. The 
relation of the rocks at the west end of the tunnel is of much more impor- 
tance and the explanation not easy without assuming a fault. It will be 
noticed by Profile in, PI. v, that the west edge of the trough of schist 
which runs along the west slope of the mountain lies at the tunnel level, con- 
siderably west of its position at the surface, so that the band of white gneiss 
lying in the tunnel west of the schist seems to lie on top of it at the surface. 
It should be remembered that this band of schist and gneiss west of it have 
been traced many miles side by side to the south point of the great fold, 
where they curve together to the east and are found in conformable contact 
and even transition with each other. It is therefore impossible to explain 
their general relations by a fault, but there may be a fault separating them 
for a short distance here or else an overturned fold in the western gneiss 
curving far back to the east, like the great Grlarus fold.^ It would be impos- 
sible fully to explain by words the structure of the east to west striking- 
gneisses just south of the west corner of the main fold. If a piece of cloth is 
worked into a number of parallel folds or plaits and one-half of the cloth bent 
around at right angles to the former general trend of the plaits, we get just 
the series of transverse folds which exist on the mountain. The sections of 
the Alps given by Heim show folding of equal complication in younger 
rocks. A model would be the proper means of representing this structure. 
One result of this work important to future investigation in the regions 
of crystalline rocks is that it shows the possibility, by proper methods of 
work, of determining much of the stratigraphy of these rocks, improbable 
as it may seem at first sight. The gneisses of the Green mountains are 
just as susceptible to stratigraphic investigation as the unaltered sediments 
of the Appalachians, but the problem is much more difficult owing to the 
secondary structures produced by metamorphism. 

'Heim, "Mechauismus der Gebirgsbildung." 



HOOSAC MOUNTAIN. 107 

In the preceding pages of this chapter no reference has been made to 
earlier work in this area, because the httle recorded is hxrgely based on a 
general survey of the Green mountains and no attempt has been made to 
master the local structure in detail. 

Most geological workers haVe given their attention to the limestone 
and schists west of the axial range. Prof. J. D. Dana, who has devoted so 
many years of his life to the Taconic question, has published no decided 
opinion on the Hoosac tunnel series. The geological sections of Presi- 
dent Hitchcock and Prof. C. H. Hitchcock,^ which cross this area, are not 
sufficiently detailed for comparison in this connection. 

Ebenezer Emmons alludes to Hoosac mountain in his "Taconic Sys- 
tem."" He considers that tlie Hoosac mountain schists were primary and 
that the lower Taconic rocks (Mount Grevlock) were derived from them — a 
theory by which he explains the close lithological similarity which he had 
observed between the two rocks. It is evident how inadequate this theory 
is to explain this resemblance when we remember that in the albite schist, 
for instance, common to both series, the albite crystals are metamorphic in 
both rocks. 

Emmons also describes (p. 120) the contact of conglomerate and gneiss 
on Clarksburg mountain, north of Williamstown. 

President E. Hitchcock ^ regards as primary the Hoosac mountain lime- 
stones at the base and part of the rocks further west. He also speaks of 
the transitions between quartzite and gneiss. 

Prof. C. H. Hitchcock * places a fault between the limestone at the west 
portal of the tunnel and the Hoosac mountain gneiss. 

In the writings of Prof. J. 1). Dana on the Taconic rocks there are a 
few allugjons to the Hoosac mountain region. He speaks of the Stamford 
granite as "an undoubted Archean area,"^ but this seems to be based on 
lithological characters. He says,® "there is some reason for making Hoosac 
mountain Cambrian." 

' Geology of Massachusetts, 1841. Geology of Vermont, 1861. 

'' Agricultural Report, New York, p. R3. 

^ Final Report, Geology of Massachusetts, p. 577 et seq. 

^Geology of Vermont, p. 597. 

s Amer. Jour. Sci., vol. 33, 1887, p. 274. 

6 Ibid., p. 410. 



108 GREEN MOUNTAINS IN MASSACHUSETTS. 

No detailed geological study of the Hoosac tuuuel seems to have been 
published, which is remarkable considering the importance of this engineer- 
ing work and the number of experts who examined it when in construction. 

In the reports of Profs. James Hall and T. Sterry Hunt as experts^ the 
general distribution of the rocks in the 'tunnel is correctly given. Prof 
Hall noticed the transition from white gneiss to granitoid gneiss at the west 
edge of the latter rock, and also speaks of the micaceous gneiss at the west 
portal "resting against or upon the limestone," an exposure no longer visible. 

' Massachusetts House Document No. 9, January, 1875, Appendix. 



PLATE VII. 



109 



PLATE VI I. 

A. Fine grained white gneiss (Vermont formation) from western slope Hoosac mountain. From 
a micropLotograph. Polarized light, x 33. 

In the lar"-e feldspar twin u. the line of twinning is oblique to the external planes of the crystal. 
The little black or white round spots in it are grains of quartz which lie roughly in lines parallel to 
the lines of arrangoraput of the quartz, feldspar, and mica outside. 

B. Gneiss (Vermont formation). Dump Hoosac tunnel. From a microphotograph. Polarized 
light, X 33. 

A large crystal of microcliue (a) has been broken into five parts in the general crushiug of the 
rock, and the groundmass, composed of little grains of quartz and feldspar and some mica, crosses 
it by the cracks. 

110 



J. S. OFOLOQICAI 






A. i'inf BTaiu 



H«aaav moimUio. Fnnu 



ligh., 

A large crystal ••£/(uicroi'line (a) 
roek, and the givniyftnass, coaipo'-"'' 
it by tlio fratks 



II); is oblii{ac to tbo oxtornal plai 

i^<vhi.h ii roughly in iiuu^ iiuijUi:! lu 
nui'a <Hit.»»i^ 

cuel. FronT'lt^icrophotograph. Polari/,<(l 

tivi! parts iu th^^eueral i rushiDK <>f the 




U. S. GEOLOGICAL SURVEY 



h-ONOGRAPH XXIIl PLATE VII 





THIN SECTIONS, WHITE GNEISS. 



PLATE VIII 



111 



PLATE VIII. 

A. Fiue graiueil white gueiss (Vermont lormation). Hoosac uiountaiD. Microphotograph. Pol- 
arized light, X 33. 

Poi-phyritic feldspar twin (a) containing Inclusions of quartz and mica which are arranged 
parallel to the minerals of the groundmass outside. 

B. Albite schist (Hoosac schist). Hoosac mountain. Microphotograph. x 33. 

The large crystals of albite (a) contain inclusions of muscovite, chlorite, magnetite, and quartz. 
The gentle curving of the mica of the groundmass between these feldspars is well shown. 

112 



U, B. QEOLOQICAL SUHVEV 



MONOORAPH XX<II PU^TE VIII 





' THIN SECTIONS. " 




\. Fine graiufd wUit* j^tu-ino i v 
iiri7.t'<l light, X '■'3. 

I'orphyritio t'«lilsi)ar twin (a) containinfr inclusions of quart)', ami iiiioa which are arranged 
)>art< ■.I'." 

^ ■ ■ ^ 13. 

I ^<r ^r\)' '■ 'I'Njf' ■ '""ifiiefit^, ami quartz. 

'I'll" ^ jT \ ui - •yi • i-w rki3|i.imii^wcll abown. 

llii 




U. S. GEOLOGICAL SURVEY 



MONOGRAPH XXIIl PLATE VIII 





THIN SECTIONS, WHITE GNEISS AND ALBITE-SCHIST._ 



PLATE IX. 



MON XXIII 8 ' ii>^ 



PLATE IX. 

A. "Amphibolite." Diorite dike. Hoosac moiiutain, south of Cheshire. Microphotograph, x 33. 
Crystalloids or grains of plagioclase feldspar (a) and of brown hurnblende (fc) are seen around 

the edge of the figure. In the center we have an aggregate of irregular patches of secondary feld- 
spar, green hornblende, epidote, etc., forming a confused aggregate, little veins of which are seen to 
penetrate the feldspars or pass between them. 

B. Amphibolite. Mount Holly, Vermont. Microphotograph; polarized light x 33. 

The large black areas are a deep greenish-brown hornblende, surrounded by a fringe of light 
green hornblende. This shows best in the crystal in the center («) with the fringe (b). The portion 
between the black crystals is an aggregate of epidote prisms, masses of green hornblende, and 
feldspar. 

114 



M ■'NO.j'^APH 





THIM SECTIONS, DIORITE AND AlllWl»BOL(TE. 




A. "Amphibolite.' I 
Crystalloids or graiiiM n 

the eUg«< of the ';.'"■■ <• > 
spar, green boi ; 
penetr.'ite <'■ 

B. A" 
T' lack 

pretM iile. This shd 

botwcoi. Uiu l)lack cryst 
)'<-l<l«j'ar. 



■oil) , Veri: 

i\ lib ;ir>' ;i il 



Mi'To|ihotograph, X 33. 
■ ndc (fc) are secu around 
.. |i:itch<4 of secondary t'vhl- 
tle veiiie of >vlii(h are iu<en lu 



)hol< igi .ipti ; polari/ed Uttht X 'X'- 

le, siirroiiDdod by a frmgi- of light 

ith tlic friiij.r (I)). The portion 

epidutc jiriouiE, iiiasftiw of green huniblolidu, and 




U. S. GEOLOGICAL SURVEY 



MONOGRAPH XXIII PLATE IX 




THIN SECTIONS, DIORITE AND AMPHIBOLITE. 



PLATE X. 



115 



PLATE X. 

A. Qiiartzite-congloiiierate (Vermont form.ation). Stone hill, Williamstown, Mass. Microphoto- 
graph; polarized ligbt, x 27. 

The shadowy area filling the left half is one of the masses of crushed blue (juartz which shows the 
so-called "wavy" extinction in polarized light. At the top it is seen passing into the quartz mosaic of 
the "gronndmass." At the bottom and lower right side a crystal of microcline has been faulted 
several times and the fine quartz of the groundmass penetrates it. 

B. Crumpled metamorphic conglomerate (Vermont formation). Hoosac mountain, bluffs south of 
Spruce hill, near that of Fig. 17. About one-eighteenth natural size. 

These pebbles are grannlitic and by pressure have been gently crumpled. This figure represents 
the transitional form between the conglomerate and tlie white gneiss; in the latter the grannlitic 
lenses remind us of pebbles, but they have lost their shape. 

116 



U. S. GEOLOGICAL SURVEY 



MONOGRAPH XXIII PLATE X 





QUARTZITE CONGLOMERATE AND CRUMPLED METAMORPHIC CONGLOMERATE. 



PLATE XI. 



117 



PLATE XI. 

A. Looking north over the crest of Hoosac mountain from the northern end of the granitoid 
gneiss (compare PI. v., Profile ix), showing the outcropping edges of the northerly dipping (pitch- 
ing) beds of conglomerate gneiss and albite schist. From a drawing by Josiah Pierce, jr. 

B. Profile of Hoosac mountain from Spruce hill southward, looking west. 

This includes the contact of all three formations — granitoid gneiss (Stamford gneiss), conglomerate 
(Vermont gneiss), and albite schist (Hoosac schist). The northerly pitch of the axis and consequent 
overlay of the formations to the north shows plainly in the long gentle northward slope and sharp 
bluffs to the south. The rounded granite topography of the coarse gneiss is also in marked contrast 
with the serrations produced by conglomerate and schist. Cf. Plate v, Profiles ix and x. 

118 



EREATUM. 
Plate XI.— Legends to Figs. A and B sliould be transposed. 



U, S. GEOLOGICAL SURVEY 



MONOGRAPH XXHI PLATE 3 







5^- "i ^^ li^rii 













;%. 



I ,- «si^>^-5H 



-■:*.«5rwt.-"-'' 



"•^^i 



A. VIEW NORTH OVER CREST OF HOOS«C MOUNTAIN. 




jl^/^- ^'^^Jf ll^^v^fe- 



B. PROFILE OF HOOSAC MOUNTAIN FBOM SPRUCE HILL SOUTHWARD, LOOKING WEST. 



P^RT III. 



MOUNT GREYLOCK: 

ITS AREAL AND STRUCTURAL GEOLOGY 



By T. n'ei.so:n d^le. 



119 



CONTENTS. 



Page. 

Outline of tliis paper 125 

Historic 131 

Physiographic 133 

Structural 136 

Types of structure 138 

Correlation of cleavage aud stratification 155 

Pitch 157 

Structural priuciples 157 

Structural transverse section? 158 

Transverse section G 160 

Transverse sections H, I 166 

Transverse sections A-F, J -0 169 

General jjitch of the folds 175 

Longitudinal sections 175 

Longitudinal section P 175 

Longitudinal section Q 176 

Longitudinal sections R' . R" 177 

RpsumtS, structural 177 

Lithologic strat'graphy 179 

Vermont formation 179 

Stockbridge limestone 179 

Berkshire schist 180 

Bello wsiiipe limestone 180 

Grey lock schist 180 

Petrography 181 

The Vermont formation 181 

The Stockbridge limestone 181 

The Berkshire schist 182 

The Bellowspipe limestone 184 

The Greylock schist 186 

Thickness 188 

Geologic age 189 

R<!sum6, lithologic stratigraphy 190 

Areal and structural 191 

Relations of geology to topography 192 

Appendix A : Stone hill near Williamstown 197 

Appendix B : New Ashford 202 

121 



ILLUSTRATIONS. 



o 



Page. 

Pl. XII. Mount Greylock, eastern side 130 

XIII. Mount Greylock, western side 132 

XIV. Southern summit of Mount Greylock 134 

XV. Southern side of Mount Greylock 136 

XVI. Southern end of Ragged mountain 160 

XVII. The north-south part of Hopper 192 

XVIII. Greylock sections A, B, C, D 

XIX. Greylock sections E, F 

XX. Greylock sections G, H, I 

XXI. Greylock sections J, K, L, M 

XXII. Greylock sections N, O -• 

XXIII. Greylock longitudinal sections P, Q, R 

Fig. 30. Mount Greylock, north-northwestern side 136 

31. Albitic sericite-schist in contact with limestone 138 

32. Sericite-schist with two foliations, in contact with limestone 139 

33. Sericite-schist; specimen with two foliations 139 

34. Thin section illustrating origin of cleavage 140 

35. Sketch of ledge south of Sugarloaf ; cleavage in both limestone and schist 140 

36. Limestone block with cleavage, Sugarloaf 141 

37. Limestone ledge with Cleavage, east of Sugarloaf 141 

38. Weathered limestone from East mountain 142 

39. Polished surface of limestone shown in Fig. 38 142 

40. Weathered limestone with mica in cleavage planes 143 

41. Specimen of sericite-schist showing stratifioatiou and cleavage. Bald mountain 144 

42. Specimen of sericite-schist showing only cleavage, Symouds peak 144 

43. Section of specimen shown in Fig. 42 145 

44. Section of specimen of sericite schist, top of Mount Greylock 145 

15. Microscope drawing of sericite schist, top of East mountain *- 146 

46. Specimen of sericite schist one-fourth mile south of Mount Greylock 147 

47. Diagrams showing relation of quartz laminse to cleavage 148 

48. Lodge of sericite-schist, junction of Gulf and Ashford brooks 148 

49. Part of ledge shown in Fig. 48 149 

50. Section of sericite-schist with quartz lamina, from Bald mountain 150 

51. Ledge of mica-schist in Readsboro, Vermont, with quartz in both foliations 151 

52. Sericite-schist with two cleavages, Goodell hollow 152 

123 



124 tLLtfSTRATIOKS. 

Page. 

Fig. 53. Section of sericite-schist, one-fourth mile soutli of Greylock top 153 

54. Sericite-schist, one-fourth mile southwest of Greylock top 154 

55. Diagram showing fault between schist and limestone 154 

56. Section of sericite-schist, Bald mountain spur 155 

57. Diagrams showing relation of slip cleavage to stratification, clips opposite 156 

58. Quartz lamina' in schist, west side of Deer hill 156 

59. Diagrams showing relation of slip cleavage to stratification, both dips east or west 157 

60. Minor pitching limestone folds 157 

61. Cross-section G 160 

62. Section of syncline at south end of Ragged mountain 161 

63. Cross-section H 166 

64. Cross-section 1 166 

65. Cross-sections A, B 169 

66. Cross-section F 171 

67. Cross-sections J, K, L 172 

68. Structure of schist on south side of Saddle Ball 173 

69. Cross-sections M, N, O - 173 

70. Structure iu schist west of Cheshire reservoir 174 

71. Longitudinal sections P, Q, R 175 

72. Continuity of the folds on the Greylock sections 178 

73: Albitic sericite-schist, typical Greylock schist 188 

74. Outline sketch of Round rocks 194 

75. Sketch of Greylock mass from the southwest 195 

76. Cross-sections S, T, U, Stone hill 198 

77. Sketch of protruding limestone anticline 202 

78. Diagram map of Quarry hill. New Ashford ~- 202 

79. Cross-section of Quarry hill, New Ashford 203 



OUTLINE OF THIS PAPER. 



Mount Gi'6ylock, or Saddle mouiitain, in northwestern Massachusetts, has been 
studied off and on by geologists for seventy years. The literature is given on p. 131. 
The general synclinal structure of the mountain is well known. This description is 
based upon the new topographic map of the U. S. Geological Survey, and upon 
the results of recent orographic science. Mr. J. Eliot Wolff has done the petrographic 
work. 

The mountain consists mainly of one central and two lateral subordinate ridges, 
all trending about north-northeast to south-southwest. With its spurs it forms a 
topographic unit and measures 16^ miles in length and averages about 3^ in width. 
Its aspects from the north, south, east, and west are described on p. 134 (Pis. xii, 
xiii-xv). Tlie "saddle" is formed by a depression in the south wiBsterly bend of the 
central ridge, between Greylock summit (3,505 feet) on the north and Saddle Ball 
(3,300 feet) on the south. These are about 2 miles apart, and the lowest part of the 
saddle is 605 feet lower than Greylock summit. 

Structural. — Tlie rocks are all metamori^hic and of few kinds, crystalline lime- 
stone, quartzite, and schists. The key to the structure is in the distinction between 
cleavage foliation and stratification foliation. The principal recent and oldei- liter- 
atui-e of that subject is given on p. 137. Thepiienomena of cleavage and stratifica- 
tion and pitch, as they occur on Greylock, are illustrated by ten typical cases. These 
lead to the adoption of the following structural principles: I. Lamination in the 
schist or the limestone may be either stratification foliation or cleavage foliation or 
both, or sometimes, in limestone at least, " false bedding." To establish conformability, 
the conformability of the stratification foliation must be shown. II. Stratification 
foliation is indicated by: (a) the course of minute but visible plications ; (b) the course 
of the microscopic iilications; (c) the general course of the quartz laminiB whenever 
they can be clearly distinguished from those which lie in the cleavage planes. III. 
Cleavage foliation may consist of: («) planes produced by or coincident with the 
faulted limbs of the minute plications; (b) planes of fracture, resembling joints on a 
very minute scale, with or without faulting of tlie plications; (c) a cleavage approach- 
ing slaty cleavage, in which the axes of all the particles have assumed either the 
direction of the cleavage or one forming a very acute angle to it, and where stratified' 

125 



126 GREEN MOUNTAINS IN MASSACHUSETTS. 

tioii foliation is no longer visible. lY. A secondary cleavage, resembling a minute 
jointing, occurs in scattered localities. V. The degree and direction of the pitch of a 
fold are often indicated by those of the axes of the minor plications on its sides. VI. 
The strikes of the stiatiflcation foliation and cleavage foliation often differ in the same 
rock, and are then regarded as indicating a pitching fold. VII. Such a correspondence 
exists between the stratification and cleavage foliations of the great folds and those 
of the minute plications that a very small specimen properly oriented gives, in many 
cases, the key to the structure over a large portion of the side of a fold. 

On these principles twelve com^ilete and three partial transverse sections have 
been constructed across the Greylock mass (Pis. xviii-xxii). These show that the 
range consists of a series of more or less open or compressed synclines and anticlines, 
which, beginning near North Adams, increase southerly in number and altitude with 
the increasing width and altitude of the schist area, and then, from a point about a 
mile and a half south of the summit, begin to widen out, and to diminish in number 
and height until they finally pass into a few broad and low undulations west of 
Cheshire. Between that point and the villages of Berkshire and Lanesboro the folds 
become sharper and more compressed, and the schist area rapidly narrows, termi- 
nating within a short distance of Pittsfield. The two most comprehensive and best 
substantiated of these sections (G and I) begin near South Adams, cross the central 
ridge north and south of the summit, then follow the two great western spurs, and 
end near South Williamstown. The sections are described on p. 160, the first two in 
some detail. The section lines on the map (PI. i) and the epitomized sections in Fig. 
72 on p. 178 show the relations of the fifteen sections to each other. 

Resume, structural. — Mount Greylock with its subordinate ridges is a synclinorium 
consisting in its broadest portion of ten or eleven synclines alternating with as many 
anticlines. While the number of these minor synclines is so considerable at the sur- 
face, in carrying the sections dowuwai'd they resolve themselves chiefly into two 
great synclines with several lateral and minor ones. The larger of these two forms 
the central ridge of the mass ; the smaller one, east of it, forms Eagged mountain and 
an inner line of foot-hills farther south. The anticline between these coincides with 
the Bellowspipe notch; that on the west of the central syncline is on the west side 
of the north-south part of the Hopper. The major and central syncline is so com- 
pressed east of Symonds peak (Mount Prospect) and Bald mountain, and its axial 
plane is so inclined to the east that the calcareous strata which underlie the central 
ridge have on its west side a westerly dip. Farther south this syncline opens out, 
and all the relations become more normal. On either side of those two main synclines 
the sub( irdinaitc folds are more or less open and have their axial planes vertical or inclined 
east or west. The long undulations in the axes of these synclines are shown in four 
longitudinal sections (PI. xxiii): Section P, the eastern or Ragged mountain syncline; 
Q, the central or Greylock syncline, and E' R", portions of two of the minor synclines 
on the west flank of the mass. In each of the sections P and Q the trough bottom 



MOUNT GREYLOCK. 127 

deepens at two points. In the eastern syncliue, P, the deeper part of the northern 
depression is shown to be about under the center of Ragged mountain, while in the 
central one, Q, the deeper part of the northern depression seems to be about 2 miles 
farther south, between Greylock and Saddle Ball and near Greylock summit. The 
northern side or edge of this great double trough is at the extreme north end of the Grey- 
lock mass ; section Q begins at Clarksburg mountain, and its southern edge is between 
7J and 8J miles distant, near Round Rocks and on the southeast spur of Saddle Ball. 
South of these main troughs is another pair, the centers of which lie west of Cheshire 
reservoir. To the west of these two long axes the mountain mass is made up of 
numerous minor folds, which do not show the continuity seen in P and Q. It will be 
seen that the direction of these two main synclines represented by P and Q is north- 
northeast by south-southwest, thus nearly parallel with the direction of the valley 
lying between the Clarksburg granitoid gneiss mass and Hoosac mountain, and that 
at the south end they converge and perhaps unite in tke narrow schist ridge 
between Berkshire and Lanesboro villages. Traversing the folds of this canoe-like 
complex synclinorium is a cleavage foliation, sometimes microscopically minute, dipping 
almost uniformly east. This cleavage foliation is distinct from the "slaty cleavage," 
early described by Sedgwick, Sharpe, and Sorby and reproduced experimentally by 
Tyndall and Jaunettaz, and consists sometimes of a minute, abrupt, joint-like fractur- 
ing of the stratification laminsB, but more usually of a faulting of these laminae as the 
result of their extreme plication — a mode of cleavage ("Ausweichungsclivage") so 
well described by Heim and recently reproduced in part by Cadell by a slight 
modification of the experiments made by Prof. Ali^house Favre, of Geneva, in 1878. 
(See foot-notes, p. 137.) This slip cleavage, when carried to its extreme, results in a 
form of cleavage very much approaching, although not identical with, slaty cleavage. 
To the unaided eye all traces of stratification are lost, and even under the microscope 
they are so nearly lost as to be of no avail in determining the dip. This and the 
regular slip cleavage often occur in close proximity. 

Lithologic stratigrapliy. — There are five more or less distinct horizons in the Grey- 
lock mass. The following descriptions are based upon Mr. WolfFs petrographic deter- 
minations, beginning above: 

The Greylock schist (Sg). Muscovite (sericite), chlorite, and quartz schist, with or 
without biotite, albite, magnetite, tabular crystals of interleaved ilmenite and chlorite, 
ottrelite, microscopic rutile, and tourmaline. Thickness, 1,500 to 2,200 feet. Part of 
Emmons's pre-Cambrian or Lower Taconic No. 3 ("talcose slate"), Walcott's Hudson 
River (Lower Silurian). 

BeUoicspipe limestone (Sbp). Limestone more or less crystalline, generally mica- 
ceous or pyritiferous, passing into a calcareous schist or a feklspathic quartzite, or a 
fine-grained gneiss with zircon and microcline, in places a noncalcareous schist. The 
more common minerals are graphite, pyrite, albite, microscopic rutile, and tourma- 
line; rarely, galena and zinc blende. Thickness, 600 to 700 feet. Par£ of Emmons's 



128 GltEEN MOUNTAINS IN MASSACHUSETTS. 

pre-Cambi'ian or Lower Taconic No. 3 ("talcose slate"), Walcott's Hudson Eiver (Lower 
Silurian). 

The Berkshire schist (Sb). Schist like the Greylock schist, but more frequently cal- 
careous and i)lumbaginoiis, especially toward the underlying limestone (€Ss) ; thick- 
ness, 1,000 to 2,000 feet. Part of Emmons's pre-Cambrian or Lower Taconic No. 3 
(" talcose slate"), Walcott's Hudson Eiver (Lower Silurian). 

The tStocTchridge limestone (■£?&). Limestone, crystalline, in places a dolomite, 
quartzose or micaceous, more rarely feldspathic, very rarely fossiliferous. Galena 
and zinc blende rare. Irregular masses of iron ore (limonite) associated sometimes 
with manganese ore (pyrolusite). Thickness 1,200 to 1,400 feet. Emmons's pre-Cam- 
brian or Lower Taconic No. 2 ("Stockbridge limestone"), Walcott's Hudson River 
(Lower Silurian). 

The Vermont formation {-Cv). Quartzite, cropping out in the Greylock area only 
once, but probably underlying the entire mass. Thickness, 800 to 900 feet, Emmons's 
pre-Cambrian or Lower Taconic No. 1 ("granular quartz"), Walcott's "O/ewei^ws" 
(Lower Cambrian). Total thickness of the series, 5,000 to 7,200 feet. 

The estimates of thickness are based upon the sections. The difference in the 
estimates arises partly from the varying amount of thickening in plication. The 
actual thickness is probably less than the minimum figures given above, and possibly 
much less. The maximum thickness of the entire series does not exceed the minimum 
thickness attributed to the Lower Silurian in the Appalachian region. See page 190 
for a tabular arrangement of these results. 

Areal rjeology. — The accompanying geographic map of Greylock and the adja- 
cent masses presents a great body of the Berkshire schist almost surrounded by 
the underlying Stockbridge Umestone. The Berkshire schist sends out tongues, cor- 
responding to synclines, into the Stockbridge limestone area. There are also reenter- 
ing angles of limestone in the schist area, corresponding to anticlines. There are 
isolated schist areas which are more or leas open synclines, and isolated limestone 
areas which are compressed anticlines protruding through the overlying schist, 
exposed by erosion. These relations recur between the Bellowspipe limestone (Sbp) 
and the Greylock phyllite (Sg), but the limestone area southwest of Cheshire appeara 
to be a syncline. 

Relation of geology to to})ography. — The physically and chemically more resistant 
schists form the more elevated portions and the steeper slopes, while the broad valleys 
and gentler undulations about the mountain generally correspond to limestone areas. 
The limestone and calcareous schist of the Bellowspipe limestone horizon consti- 
tute the benches of agricultural land high up on the sides of the mountain and the 
Notch ; and to the presence of this rock also, together with a northerly pitch, is due 
the deep incision in the central crest between Saddle Ball and Round rock. (See sec- 
tion Q and PI. xiii and Fig. 74. The north to south part of the Hopper (PI. xvii) 
is due to the trend and upturned edges of the calcareous belt, and possibly also to 



MOUNT GKEYLOCK. 129 

the minor anticline on the west side of this part of the Hopper. The deep east to 
west incisions on both sides of the mountain are the results of erosion crossing the 
strike, while the great spurs on the west side are portions of the original mass left 
by this erosion. The saddle between Greylock summit and Saddle Ball seen from 
the south (PI. XV) is due to the central syucline of the mass (Sections I and K). 
The broader saddle seen from Mount Equinox on the north-northwest (Fig. 30, 
p. 130) is due to the great trough in the central syncline (Section Q). The center of this 
trough is the deepest part of the entire syuclinorium. 

In Appendix A, Stone hill, near Williamstown, and in Appendix B, New Ashford, 
are described in some detail. The former is accompanied by three transverse sec- 
tions, S, T, U, which are crossed by the longitudinal section R', from which it appears 
that a subordinate syncline passes through Stone hill and Deer hill, whence it prob- 
ably continues southward through East and Potter mountains. The relation between 
Stone and Deer hills is analogous to that between Clarksburg mountain and Grey- 
lock. 

MON XXIII y 



I. GEOLOOICAL SURVEt 



Chc^w'0^ 



V/uGz/ioa 



,S-^lUCU/iX 



uV/iiC^j^ZaoIits 



'JUfu^jxeJi'lt. 



^.Atican^ 




From a point on Hoomc mountain about 4 inile* soulh of NortK Atlamt anil 500 feet shove Hooiic nvei, showing Ihv nia» from North Adams 10 Chethiie, 1 1 



MOUNT GHEYLOCK EASTERN SIDE. 

iiles. In Ihu northern half, the high bench of arable la..d (marked by 2 b.ids», Bellowspipe limeslone, sepatated ('omtho Hooiic valley by a steep of«a of the Berkshire schis! 
foothills of Berkshire schist separated from the central mass by areas of Bellowspipe limestone F'"m Photographs 



1 bench the Ragged mountain mass, Greyloek schist, separated from the central ridge by the Notch; m the southerr. half the 



MOUNT GREYLOCK: ITS STRUCTURAL AND AREAL GEOLOGY.' 



By T. Nelson Dale. 



HISTORIC. 



Mount Greylock, or Saddle inountaiii, has been an object of interest to 
geologists for seventy years. The most important work in structural and 
areal geology that has been done on the mountain is that of Prof Chester 
Dewey (1817-1829), Prof Ebenezer Emmons (1833-1855), Prof Edward 
Hitchcock (1856-1861), and Prof James D. Dana (1871-1887.) Prof Em- 
mons built upon and extended the investigations male by Prof Dewey. 
In the writings of Profs. Dewey, Emmons, Hitchcock, and Dana," the 
general boundaries between the limestone of the Hoosic and Green river 
valleys, and the schists of Greylock and Deer hill, and the qiiartzite of 
Stone hill are given. The synclinal structure of the Greylock mass, and 

' A report to Prof. Raphael Pumpilly, in charge of tlio Archean Division, covering field work dune 
nndcr his direction in the sninmers of 1886, 1887. and part of 1888, by the writer, with the assistance 
duriua- 1886 and part of 1887 of Mr. Wm. H. Hobbs. 

- Amos Eaton : Index to the Geology of the Northern States. 1818. 2d ed. 1820. 

Chester Dewey : Sketch of the mineralogy and gtology of the vicinity of Williams College, 
Willianistown, Massachusetts (in a letter to the editor of tlni American .Journal of Science, dated 
January 27, 1819, witli a geologic map and .section of the northwest part of Massachusetts). Am. 
Jour. Sci., ser. i, vol. 1, 1819, p. 337. 

Chester Dewey : Geological section from the Taconick range in Williamstowu to the city of 
Troy on the Hudson. Am. Jour. Sci., ser. i, vol.2, 1820, p. 24fi. 

Amos Eaton: Geological and agricultural survey of the district adjoining the Erie canal. 
1824. (This includes a section from Hoosac mountain, Savoy, to the Hudson at Troy. It is repro- 
duced in a paper by C. D. Walcott in the Tenth Annual Kept., U. S. Geol. Survey, 1888-89, p. 525.) 

Chester Dewey : A sketch of the geology and mineralogy of the western jiart of Massachusetts and 
a small part of the adjoining states (with a geologic map of tlie county of Herkshire, Massachusetts, 
and of a small part of the adjoining states). Am. .lour. Sci., ser. I, vol. 8, ])art 2, 1824, ]). 1. 

Amos Eaton: A geological nomenclature for North America, founded uimn surveys taken under 
the direction of the Hon. Stephen Van Kens.selaer. Albany, 1828. 

Chester Dewey : A general view of Berkshire county, forming pai 1 1 of " A history of the county of 

131 



132 GKEEN MOUNTAINS IN MASSACHUSETTS. 

the relation of the Hmestoiie to the schist were pointed out by Profs. Hall 
and Emmons, and confirmed by Profs. Hitchcock and Dana, and the com- 
plex character of that syncliue was recently conjectured by Prof. Dana. 
Moreover, scattered through the writings referred to, are a number of 
important observations on portions of the mountain, to which reference will 
be made in proper place. 

Of these writings, those of Profs. Emmons and Dana include the 
Taconic question, into the consideration of which the structural and areal 
geology of the Greylock mass partly enters. Notwithstanding the time 
that has elapsed since a geologic hammer was first applied to Mount Grey- 
lock, and notwithstanding the number and ability of the geologists who 
have lived and worked in its vicinity, little has been accomplished beyond 

Herkshire, Massachusetts, by gentlemen iu tho county, clerijymcn, and Laymen." Pittsfield, 1829 (p. 
190, "Geology," and "a geological map of the county of BerksUiro, Massachusetts, audof asmall part 
of the adjoining states, 1824"). 

Edward Hitchcock : Report on the geology, mineralogy, botany, and zoology of Massachusetts. 
First and second editions, Amherst, 1835. 

Edward Hitchcock : Final report on the geology of Massachusetts. Amherst and Northampton, 
1841. 

Ebeuezer Emmons : Taconic system, forming chap, vii of the Geology of Now York, part u. Nat. 
Hist, of N. Y., part iv, Albany, 1842. 

Ebenezer Emmons : The Taconic system, based on observations in New York, Massachusetts, 
Maine, Vermont, and Rhode Island, Albany, 1844. 

Ebenezer Emmons: The Taconic system, forming chap. v. of vol. 1, of the Agriculture of New 
York. Nat. Hist, of N. Y., part v, Albany, 1846. 

Ebenezer Emmons : American Geology, vol. 1, part ii, Albany, 1855. 

Edward Hitchcock : Report on the Geology of Vermont : descriptive, theoretical, economical, and 
scenographical. Proctorsville, Vermont, 1861, vol. 1, p. 255, vol. 2, p. 595, pi. XV, fig. 5. 

James D. Dana : On the quartzite, limestone, and associated rocks of the vicinity of Great Bar- 
rington, Berkshire county, Massachusetts. Am. .Jour. Sci., ser.iil, vol. 6, 1873, p. 273. 

James D. Dana: An account of the discoveries iu Vermont Geology of the Rev. Augustus Wing. 
Am. Jour. Sci., ser. m, vol. 13, 1877, p. 347. 

James D. Dana : On the relation of the Geology of Vermont to that of Berkshire. Am. Jour. Sci. 
ser. Ill, vol. 14, 1877, pp. 41, 261-263. 

James D. Dana: Note on the Age of the Green mountains. Am. Jour. Sci., ser. in, vol. 19, 1880, 
p. 191. 

James D. Dana : On Taconic rocks and stratigraphy, with a geological map of the Taconic region. 
Part II. Am. Jour. Sci., ser. iii, vol. 33, May, 1887, p. 405, 410. 

James Hall : Section from Petersburg, New York, across Greylock to Adams, the basis of remarks of 
his at a meeting of the American Association of Geologists and Naturalists, between 1839-1844, both 
unpublished. See Am. Jour. Sci., ser. ill, vol. 28, 1884, p. 311. " Prof. James Hall on the Hudson river, 
age of the Taconic slates." 



s i 



CO c 



o 



S a " 

o o m 



5' * 

3 D- 




MOUNT GEEYLOCK. 133 

what is above outlined, probably because of the wide reach of territory 
covered by the Taconic belt, and the overshadowing importance of the 
stratigraphic relations on either side of it, as well as the imperfection of the 
topographic maps hitherto published, and jjossibly because of the somewhat 
rugged character of portions of the mountain. 

The raisons d'etre of this report are: That Mount Greylock, in itself, 
offered one of the best fields for the study of the relations of the Taconic 
rocks to each other, and that sections across it, when extended eastward, 
northward, and southward, cut the underlying and older rocks where the 
latter were being studied in detail by the same division of the U. S. Geo- 
logical Survey; that careful work here would aid in unraveling the geology 
farther west in eastern New York; that the geologic field work has been 
based upon a more correct topographic map ; that the observations made 
have been very numerous (in all, 1,850), and have been carefully recorded 
on such a map ; that the work has been done in the light of recent advances 
in orographic science, notably of the special investigations of Swiss and 
Norwegian geologists into the structure of metamorphic rocks; that a large 
collection of specimens has been gathered, illustrating principles of struc- 
ture, from which large thin sections have been prepared for microscopic 
study; that the photographic camera has been freely used in the field as 
well as the study, and that the lithologic specimens gathered in the course 
of this structural work have been subjected to optical examination by a 
petrographer. Prof Pumpelly has also brought his wide experience and 
critical judgment to bear upon the supervision of the entire work. 

PHYSIOGRAPHIC. 

The Tiorthern third of the western portion of Massachusetts is marked 
by three main parallel mountain masses having the trend common to the 
Appalachian system. The most westerly is the Taconic range, the crest of 
which divides the states of New York and Massachusetts; the most easterly, 
situate about ten miles east of the New York line, is Hoosac mountain, and 
the central one is Mount Greylock. East mountain and Potter mountain 
together constitute a fourth but subordinate mass, connecting the Greylock 
mass with the Taconics farther south. 

Mount Gi'eylock with its spurs forms a topographic unit It is sep- 



134 (IKEEN MOUNTAINS IN MASSACHUSETTS. 

arated on the nortli from Clarksburg or Bald mountain, a projection of the 
Green mountain range, by an east-west valley, through which the Hoosic 
river turns on its way to the Hudson; and from that point the Greylock 
mass rises 2,700 feet in a distance of 5 miles to an altitude of 3,505 
feet above sea level, and thence descends more or less gradually for 11.^ 
miles in a general south-southwestern direction, dying out in gentle undu- 
lations Avithin about 2^ miles northeast of the town of Pittsfield. On the 
east it is separated from the Hoosac range by the alluvial and terraced 
valle}'' of the Hoosic, while on the northwest it is divided from the Taconics 
by the broad and picturesque valley of Green river, which flows into 
the Hoosic at Williamstown. On the west and southwest it is separated 
from East and Potter mountains by the valleys and glens through which 
flow the headwaters of Green river on the north and of the Housatonic on 
the south. 

The aspect of Mount Greylock from a point about 4 miles south of 
North Adams, on the flank of Hoosac mountain, embraces the eastern side 
of the mountain almost in its entire extent (PI. xii), and shows a central 
mountain mass, of elongated but symmetrical form, with subordinate masses 
of similar shape and parallel trend, steep, rocky, wooded, and separated 
from the central ridge by areas of gently sloping cultivated land. This alter- 
nation of wood and meadow land, and the variety of form and color which 
it produces, are striking features in the landscape, and, as will be shown 
farther on, have nmch geologic significance. 

The western aspect of Mount Greylock, from a point on the Taconic 
crest west of South Williamstown, forms a marked contrast to the eastern 
(PI. xiii). Hei'e the central crest is seen to descend rapidly about 2^ miles 
south of the summit, and then to rise a few hundred feet again. This inci- 
sion in the crest is better shown in Fig-. 74. Two powerful buttress-like 
spurs project from the central mass westwardly for over 2 miles. Their 
summits are but 900 feet lower than that of Greylock. The northerly 
spur, Mount Prospect, or Symonds peak, is separated from the southerly 
one. Bald mountain,^ by a deep east- west cut, called the "Hopper." This 
cut branches out to the east into four deep ravines, which penetrate still 

' This Halil moiiutaiii should not be ooufouuded with Clarksburg mountain, which i.s sometimes 

called bv that uaine and kiiuwii alsi) a.s Oak hill. 



U. S. GEOLOGICAL SURVEY 



MONOGRAPH XXIII PLATE XIV 



Sb 



Sbp 



^:icuijcile BaU Sg. 



Ipper .Bench- S</ 



JLoyverSericIi^ Stp- 
















SOUTHERN SUMMIT OF MOUNT GREYLOCK. 
The southern summit of the Grey lock mass (Saddle Ball), west side, from the north foot of Sugarloaf mountam. New Ashford, showing the bench of arable land due to the calcareous schist iBellowspipe limestone l and the still higher bench in the Grey lock schist formation. From a photograph. 



MOUNT GEEYLOCK. 135 

farther into the mountain, while on the west, across its mouth, lies Deer hill. 
(Compare Pis. xiii and xvii with the map, PI i.) The portion of the western 
face south of these great spurs is best seen from the north end of East 
mountain or from the north end of Sugarloaf mountain in New Ashford. 
This shows (PI. xiv), a few himdred feet below and parallel to the central 
crest, a very regular, horizontal bench over a mile in length, below which 
is a steep declivity followed by a far wider and longer bench of more or 
■less open pasture land. (See also Fig. 74, p. 194.) Below this again the 
base of the mountain is deeply cut into by a series of east and west ravines 
parallel to the Hopper. The northern one of these is known as Goodell 
hollow. 

The aspect of the Greylock mass on the south (PI. xv) from the 
north end of the Lenox mountain range (known in Pittsfield as South moun- 
tain), which is about 15 miles south of the Greylock summit, shows the pe- 
culiar saddle shape of the higher portions of the mass which render the 
name of Saddle mountain so appropriate, and so familiar tlu-oughout south- 
ern Berkshire Greylock summit (3,505 feet) and Saddle Ball (3,300 feet), 
about 2 miles apart, form the two humps of the saddle, while the inter- 
vening portion of the crest with a southwesterly bend descending to the 
2,900 feet contour forms its seat. This corresponds to internal stuctural 
features. This aspect also shows the subordinate ridges and spurs on either 
side of the mass as well as the benches on either side of its higher portions. 

The aspect of Greylock from Clarksburg mountain on tlie north shows 
the central ridge with two lateral and lower ridges : that on the east — Rag- 
ged mountain — separated from Greylock proper by the Notch ; that on the 
west, forming Mount Prospect and Bald mountain, separated from the cen- 
ter by a minor saddle, hence long ago also called Saddle mountain, which 
farther south passes into the north-south gorge continuous with the Hopper. 
From the Coast Survey station on Mount Equinox in Vermont, which is about 
35 miles north northwest, and therefore at an acute angle to the strike 
of Greylock, the saddle form of the central crest appears much broader 
(Fig. 30). On the east of it the top of Ragged mountain is seen, and on the 
west several of the subordinate masses.^ The structural significance of these 



' Prof. Edward HitcUcock in his Final Eeport on (be Geology of Massachusetts (1841, pp. 229-233) 
gave a very graphic description of Greyloclv. 



136 



(iKEEN MOUKTAINS IN MASSACHUSETTS- 



topograpliic features will be noticed at the end. The area covered by the 
mountain, as thus defined, measares 16^ miles by about 3 J ; that is, about 
63 sqtiare miles. If the short intervening range of East and Potter mount- 



OeervMts. 



Greylock SajCUUeBaU.SugarLoa/: EasfJvit. 

■^:BenchatSencl.Form.Sbp. CcUc.Sc/tlst, 




Fig. 30.— Sketch of Mount Greylock, " Saddle mountain," north-northwest side, from the TJ. S. Coast Survey station, on 
Mount Equinox, in Vermont, about 35 miles distant, shipwing the depression corresponding to the great trough in the cen- 
tral syncline, and the bench .it the soutli end of the mass, due to the Bellowspipe limestone horizon (shown by 2 birds) 
Owing to the direction of the view the mountain appears much foreshortened. 

ains be included (and structurally it belongs to the Greylock mass), the' 
mountain area would measure about 85 square miles. 

STRUCTURAL. 

This entire area consists of a few kinds of metamorphie rocks: lime- 
stone, more or less crystalline and micaceous, quartzite, and schists — 
chloritic, feldspathic, pyritiferous, plumbaginous, calcareous. In the val- 
leys, and along the lower and less inclined portions of the hills these rocks 
are covered with drift. 

The key to the geologic stracture of Mount Greylock is an under- 
standing of the relations of cleavage and stratification and the relation of 
these to the pitch of the folds.' 

There are large areas, sometimes half a mile square, where the only 
foliation presented by the outcrops is of secondary character and where no 



' Altliouj;!! Professor Eaton, iu lii.s .section of 1820, imlicates cleavage on the T.aconic range, its 
importance seems to have been overlooked by his successors iu the study of this region. 



_l ^ en 

S" s- I 



a. o- 



3 ? „ 



3 3 



«■ o X. 

5' 5- 5 

» *" " 

2- CD ^ 

o <o o 

=" — S 

(B O _. 



2 ^ 

3 "^ 



rr c 
» 3 
S 3 




(I 






i^'^ 







^ p r r *i 













J' 



MOUNT GREYLOCK. 137 

trace of stratification can be detected. As the cleavage foliation in some 
places coincides with the stratification foliation both in strike and dip, in 
others ag-rees in strike while differing in angle of dip, and in still others 
differs from it in the direction of both strike and dip, and, furthermore, as 
the marks of stratification are not infrequently subject to purely local 
changes, the whole matter is attended with much difficulty. This is enhanced 
by the absence of all outcrops over considerable areas. Satisfactory results 
can be reached only by accumulating a great number of observations, 
rejecting those which appear in the least doubtful, and by closely studying 
the relations of the remainder. As a rule, the most reliable structural data 
on Grey lock have been obtained from outcrops where two different beds were 
in visible contact, or from a series of related outcrops in all of which both 
cleavage and stratification foliations were equally manifest and discordant, or 
else from large surfaces of rock at right angles to the strike, where the general 
trend of the minor folds could be distinctly seen.' 

' The following list includes impoitant recent works bearing on the subject of cleavage: 

Theodor Kjerulf: Oni Stratifikationens Spor (traces of stratification). Kristi.ania, September, 1877. 

A. Heim: Mechanismus rter Gebirgsbildung, im Anschluss an die Keologische Mouographie der 
Toedi-Windgiillen-Grnppe. Basel, 1878. 

A. Daubree: Etudes synthetiques de g^ologie exp<irimentale. Paris, 1879. 

H. Clifton Sorby: On the structure and origin of noncalcareous stratified rocks. Quarterly 
journal of the Geological Society of T^ondon, vol. 36, 1880, p. 72. 

Ed.,Januettaz: Mdmoire sur Ics clivages des roches (schistosit(5, longrain), et sur lenr reproduc- 
tion. Bulletin dela Soci6ti$ G^ologique de France, 3rd ser., vol. 12, 1884, p. 211. 

O. Fisher: On fanlting, jointing, and cleavage. Geological Magazine, new series, decade 3, vol. 1, 
p. 205, 266, 396. London, 1881. 

A. Harker: On slaty cleavage and allied rock-strnctures, with special reference to the uiechau- 
ical theories of their origin. British Aasooiiition Report. 188.5 (1886), pp. 813-852. 

T. G. Bonney: On the inetamorphic rocks. Anniversary address. Quarterly .Journal of the 
Geological Society of London, vol. 42, p. 35. London, 1886. 

Hans Reusch : Geologische Beobachtungen in einem regional nietamorphozirten Gebiet am Har- 
daugerfjord in Norwegen. Neuos Jahrbuch fiir Min., Geol. u. Pal., V Boilage-Band, Heft I, p. 53. 
Stuttgart, 1887. 

Emm. de Margerie & Dr. Albert Heim : Die Dislocatiouen der Erdriude; Versnch einer Definition 
und Bezeichnung. Ziirich, 1888. 

Henry M. Cadell: Experiments in mountain building. Transactions of the Royal Society of 
Edinburgh, vol. xxxv, part i, third series of experiments. Feb. 20, 1888. Abstract in Nature, vol. 
37, p. 488. March 22, 1888. 

Hans Reusch : B0ramelcln og Karmoen med omgivelser geologisk beskrevne. With an English sum- 
mary of tlie contents. Kristiajiia, 1888. 

T. Nelson Dale : On plicated cleavage foliation. Am. Jour. Sci., ser. iii, vol. 43, 1892, p. 318. 

Geo. F. Becker: Finite homogeneous strain, flow and rupture of rocks. Bull. Geol. Soc. Am., vol. 
4, 1893, pp. 13-90. 



38 



GREEN MOLfNTAINS IN MASSACHUSETTS. 



Loc.602 



TYPES OF STRUCTURE. 

In order to present this matter more clearly, a few typical localities 
will here be described in some detail. 

CASE I. 

On Quarry hill, close to the village of New Ashford,^ there are several 

minor folds in the limestone and the over- 
lying schist, where the two rocks may be 
seen in contact. Fig. 31 represents the 
structure at one of these points of contact, 
locality 602 on sketch map, Fig. 78. 

The banding in the limestone, the plane 
of contact, the small quartz laminae, and the 
general, slightly undulating foliation in the 
overlying- coarse, feldsjjathic schist, all dip 
in the same direction at an angle of about 
30'^." There is little room for doubt that 



SE. 



'biri'c mica schist 



5 ft 




Fig. 31. — Diagrammatie sketch .shnwiuji al 
bitir schist iu eoDformable cnntai-t with iimlcrly 

ing cry.stamne limestone, tbo foliations of both ^j^g foHatiou iu the schist hcrc, wliatevcr its 

rocks (lipping 30^ SE. Locality 602, Quarry hill, ' 

New Asbford. cause, is parallel with the stratification, and 

that both rocks are conformable. This is the normal structure. 

T. Nelson Dale: The Rensselaer Grit Plateau in New York. Thirteenth Annual Report, U. S. 
Gcol. Survey, 1893, pp. 291-340. 

Of the older well-known works ou this subject the following are the most important: 

A. Sedgwick: On the structure of large mineral masses. Tran.s. Geol. Soc. of London, 2nd ser. 
vol. 3, 183.^, pp. fi8, 461. 

Charles Darwin: Geological observations on South America, lieing part iir of the geology of the 
voyage of the Beagle. London, 184r>. Chap. vi. Plutonic and metamorphic rooks; cleavage and 
foliation. 

Daniel Sharpe: On slaty cleavage. Quarterly .Journal, Geol. Soc. London, vol. 3, 1847, p. 74. 

Henry Clifton Sorhy : On the origin of slaty cleavage. ildinb. New Philosophical Journal, vol. 
53, 1853, p. 137. 

John Phillips: Report on cleavage and foliation in rocks, and ou the theoretical explanations of 
these phenomena. Report of British Association for the Advancement of Science, Part 1, 1856, p. .369. 

Henry Clifton Sorliy : On slaty cleavage as exhibited in the Devonian limestone of Devonshire. 
Philosophical Magazine, ser. iv, vol. 12, London, 18.56, p. 127. 

John Tyndall: On the cleavage of slate rocks. Philosophical Magazine, ser. iv, vol. 12, London, 
1856, p. 129. 

Samuel Haughton : On .slaty cleavage and the distortion of fo.ssils. Phil. Mag., ser. iv, vol. 12, 
London, 1856, )). 409. 

'See Appendix H, Kigs. 77, 78. 

■'All comjjass readings in this report are cori'ected for variation. 



MOUNT GEEYLOCK. 



139 




CA.SE II. 

About 150 feet northeast of locality G02 there are two very small folds 
in the limestone, passing into a very low southwesterly dip on the west. 
(See Figs. 32 and 78.) In the overlying 
plumbaginous schist there are corresponding 
undulations, but these ai'e compounded of 
more minute ones and crossed by cleavage 
planes. Where the plications dip 50° south- 
west the cleavage planes dip 40° to 50° east. 

Where the former dip 15° to 20° southwest Fig. 32.-Diagraramatic sketch of the north 

^ side of a lodge at locality 297, on Quarry hill, 

the latter dip 35° east, and, again, where the now Ashtowi, showing phimbaghious s.hist in 

■*- / f CD f conformalile contact with unucrlying crystal- 

former are more nearly horizontal the latter ""» "'"'^t'""". ''"'i ■'' cleavage foliation cross- 

^ ing the stratification ioliation ot the sc^hist at 

are vertical. P'ig. 33, taken fi-oni the upper 7*™"^ angles. 
portion of the section (Fig. 32), shows the relations first described. Fig. 34, 

taken from a slightly enlarged 
])hotograph of a large section 
of a specimen from the same 
portion of tlie ledge, shows 
more distinctly what is but 
slightly apparent in Fig. 33, 
namely, that the cleavage 
planes arise in a faulting along 
the shanks of tlie plications. 
In many cases tlie faulting is 
only incipient. In a specimen 
from the central part of the 
leda'c where the cleavasfe 
planes are ^'ertical they are 
simple joint-like fractures 
across the stratification folia- 
tion ftf the schist, but along one of these faulting has occurred, and the 
stratification foliation is bent about into the direction of the cleavage. 

We have here, then, a cleavage which is in jiart a microscopic joint- 
ing, in })art what Heim has called "Ausweichungsclivage"' (slip cleavage), 




Fig. 33.— Specimen in inverted position, facing south, from the 
upper part of thi- rock figured iu Fig. 32, locality 297, Quarry hill. 
New Ashford. PUiiiibagiuous schist with .a stratification foliation 
dipping southwest about 50°, crossed by a coarse cleavage foliation 
dipping 400-50° E. From a photograph. 



> See Heim, op. cit., vol. ii, ji. 5i, Gesetz 1, aurt Atlas, PI. xiv, Figs. 17, 18 ; PI. .xv, Figs. T, 8, 9, 11, 14. 



140 



GREEN MOUNTAINS IN MASSACHUSETTS. 



resulting in a coarse foliation crossing the stratification foliation at angles 
varying from 45° to 90°, and abutting against the limestone which under- 
lies th.e schist in conformable contact. 




Fig. 34. — Thin aection of a specimen from tiie upper part of tbe rock tigured in Fig. 33. locality 297, Quarry hill, Now 
Ashfurd, enlarged almost 2 diameters, showing cleavage planes arising in slight faults along the aides of the plications. 
The fractures which occurred in the preparation of the slide are mainly in tbe direction of the stratification foliation, 
which here dominates. 

CA.SK III. 

At the south end of Sugarloaf mountain, one of the subordinate folds 
of the Greylock mass, a small isolated mass of feldspathic schist over- 
lies the crystalline limestone. (See map, PI. i, locality 324 and Fig. 35.) 
Here limestone and schist are seen in contact, lioth distinctly plicated, and 




FlQ. 35.— Diagrammatic sketch of the south side of a ledge at locality 324, south foot of Sugarloaf mountain. New Ash- 
ford, showing albitio .schist in confonnable contact with underlying cryataUiue limestone, and a coarse and line cleavage 
foliation crossing the stratification foliation of both rocks. 

dipping in a general westerly direction, but really forming part of a minor 
fold. Where the stratification foliation dips jGO° west it is crossed by 
cleavage planes dipping 35° east, which in places traverse both rocks. 
The limestone a few feet away from the schist appears in thick beds. Both 
schist and limestone are traversed here and there by coarse or fine cleavage. 



MOUNT GREYLOOK. 



141 



The presence of both cleavage and stratiticatiun in limestone is also 
seen in a small mass a little north of this locality (Fig-. '66), probably 




Fig. 36.~Block of limestone <J feet liiyli ou the aoutUweat fuot of Siigarlo<it' mnuutaiu, Now AsliiVird, showiiij; a 
coar.se stratilication i'oliatiou dippiuji to tlio riy;bt, crossed by a tine cleavage foliation dipping to tlie left. Frtnu a photo, 
graph. 

detached from some part of the foot of Sugarloaf mountain, and still more 
strikingly and on a large scale on the east side of the same mountain, 
(locality 590, Fig. 37). The cleavage foliation dips here about 20^ east, 



Strat.f drpbS-W 




Fig. 37 — Sketch of the south side of a limestone ledge at locality 590, on the east side of Sugarloaf mountain, showing a 
coarsely plicated stratilication foliation dipping about 65^^ west, crossed by a cleavage foliation dipping about 20^ east. 
Area, 25X15 feet. 

and the stratification about 65° west. In some of the neighboring ledges 
only the easterly dipping foliation is visible. 



142 



GREEN MOUNTAINS IN MASSACHUSETTS. 



(_)ii tlie east side of East mountain, near the old marble quairies and 
sawmill (locality 756), there is a ledge of limestone with a thin lamination 




Fig. ;i8.— Specimen from the weathered end of a limestone ledge at locality 756. east side of East mountain, allowing a 
plicated stratification foliation dipping to the right and a cleavage foliation to the left. Photographed in inverted position. 

dipping 25° to 40° east. On a closer examination the weathered end 
of the ledge shows that this is crossed by a plicated foliation dipping 30° 




Fig. 39.— Polished surface of limestone specimen, Fig. 38, in its natural position, facing south, showing stratification 
dijiping west and cleavage east. From a iikotograiih. 

to 40° west. The presence of a little quartz in some of the stratification 
planes makes the plications project on the weathered surface. (See Fig. 38). 



MOUNT GREYLOCK. 



143 



On a polished .surface they can easily be traced. (See Fig. 39.) On the 
north side of the ridge, south of the Hopper, (locality 899) the stratifi- 
cation foliation is indicated by white calcite meandering through the 
gray limestone. At a small cave about two-thirds of a mile northeast of 
the Lanesboro Iron Company's ore bed (locality 998) the relation of cleav- 
age to stratification in limestone is also clearly seen. That the plane folia- 
tion is cleavage foliation is rendered highly probable from the usual char- 
acter and origin of such foliation'. There may be cases, however, where it 
would be difficult to decide whether the plications in limestone are due to 
"false bedding" or to original stratification. 

From all this it appears that cleavage phenomena in the Oreylock area 
affect both schist and limestone. 



CA.SE IV. 



In some loose pieces of limestone found on Quarry hill, New Ashford, 

1 




F]G. 40.— Loose piece of limestoue from Quarry hill, New Ashford, showinj: on the weathered surface laiiiiua^ of 
micaceous matter in both cleavage and stratification planes. The neai'ly hurizoDtal lamina' represent the stralitication 
foliation. From a photograph. 

both cleavage and stratification foliation are indicated by laminae of mica- 

' Cleavage foli.atiou may be .subsequently bent, but tliis rarely occur,s. See Ch. Darwin, loo. cit., 
also J. B. Jukes: Student's Manual of Geology, edited by Archibald Geikie, 3.1 ed., Edinburg, 1872, 
p. 224, 225. Dr. H. Reuscb in his Geology of the Islaiid.s of Bouimelo and karnio, etc., already cited, 
describes on p. 196, Fig. 2, and p. 408, an interesting sjiiciuien from Foien, an islet .at the mouth of the 
Hardangerfjord in Norway. The specimen figured shows both the original stratiiication foliation 
(jilicated) and the ensuing cleavage foliation (slip cleav.age), and also the secondary plication of both 
of these foliations, all on a small scale. One or two (ireylock specimens show a slight flexure of the- 
cleavage foliation. Plicated cleavage in the Taconic range at West Rutland, Vt., is described in the 
author's report on the Ren-sselaer (irit Plateau in the Thirteenth Annual Report of the Director of 
the U. S. Geological Survey, pp. 291-340. .See also A. Baltzer, op. cit. (p. 152), pi. xui, tig. 11. 



144 



GKEEN MOUNTAINS IN MASSACHUSETTS. 



ceous iiuitter wliieli })r(iject ou the weathered «urt';u-e. (See Fig-. 4U). These 
.specimens clearly indicate iutiltration and inetaniorphisni subsequent to 
cleavage. 

CASE V. 

The stratification foliation and tlie cleavage foliation are both some- 
times minute in the schist and equally dominant. Fig. 41 represents such 
a specimen from Bald mountain on the west side of Greylock. 



^"" 





'■% 



.'9S(J'S7 



/ 



Inth 




!FlG. 41. — .S]ieciineu of schist frutu lucaiity 95 uu B.-ild moun- 
tain, west side uf Greylock, nut in natural pnsitinn. showing 
both stratiticatiun and cleavage foliations somewhat minute 
and equally duiiiiuaul. Kach jiair i.f ojiposite sides of the 
hloclv is jiarallel to one of the foliations, cleavage dips to the 
loft. From a photograph. 



Flu. 42.— Specimen of schist from local- 
ity 621, north end of Mount rrospect, in 
natural position, facing south, showing 
only cleavage foliation dipping 50^ east. 



Fig. 42 represents a specimen from Mount Prospect in which only 
cleavage planes dipping 50° east are visible to the naked eye. Under a 
magnifying glass the stratihcation foliation barely appears in minute crinkles 
crossing the cleavage planes, but the cleavage foliation dominates. These 
crinkles come out more clearly in an enlarged section (Fig. 43) and indicate 
a westerly dip, which is confirmed by observations on some of the neigh- 
boring ledges, where the stratification foliation, marked by small plicated 
quartz lamina? visible to the unaided eye, dips at a high angle west. Simi- 



MOUNT GREYLOCK. 



145 



larly some of the schists on Bald mountain, near where specimen d5d (Fig. 
41) was obtained, sliow notliino- but cleavage planes, and even under the 




Fig. 43. — Tliiu stM-tion ol' part ol Mpecimeu, Fig. 42, euluri^t-d 2^ diameters, showiug a minute, plicated stratification 
foliation crossing a tine cleavage foliation. Fractures in preparing alide took place along the cleavage which here (iomi- 
nates. 

microscope barely reveal the other foliation. The structural character and 
relations of these foliations appear in Figs. 44 and 45, which show how the 
crinkling, and sometimes the exceedingly minute faulting of the small 




Fig. 44. — Thin section of .a sitcciiiien of wchist from near the top of Mount Grcylocii, enlarged 2\ diameters, showing 
the (levelopnieut of slip cleavage from tlie crinkling of the laminre of quartz and loUa of mica and chlorite. Tlie fracture 
on the right follows mainly the direction of the cleavage. From a photograph. 

laminne of quartz and folia of muscovite and chlorite of the stratification 
foliation produce cleavage planes. The schists of the Taconic range show 
these foliations on a still more minute scale, 
MON XXIII 10 



146 



GREEN MOUNTAINS IN MASSACHUSETTS. 



These facts indicate that stratiiication fohatiou aud cleavage foUation 
may be equally or uuequally dominant or microscopic. 




Fig. 45.— Microsccijiir drawing of .ibout i inch square of a tliin section of a specimen of schist from locality 741, on 
East mountain, culargcnicut :i9 diameters. The light portions are mainly quartz, the dark mainly sericite and chlorite. 
The central plication shows the development of cleavage from a slight crinkling to a complete fault. Another cleavage 
plane, about J of a millimeter to the right, contains some ferruginous matter. 

CA.SE VI. 

Frequently small lenticular masses or laminae of quartz of irregular 
thickness occur in the schists. Their form and direction are sometimes so 
irregular as to give no information as to structure, but they sometimes show 
a general parallelism either to the cleavage foliation or to the stratification 
foliation or to both. Fig. 46 represents a specimen from locality 550, about 
1,500 feet south and 500 feet below the Greylock tower. 

The specimen consists of two parts, a mass of schist about 3 inc"hes 
thick, capped by a quartz lamina about a half-inch thick, which undulates 
conformably to the general stratification foliation of the schist. The strati- 
fication foliation dips west at a very low angle, while the cleavage foliation 



MOUNT GREYLOCK. 



147 



dips 60° east. Within a space of 2 inches the schistose part of the speci- 
men shows as many as twenty cleavage planes crossing the stratification 
fohation, besides quite a number of incipient cleavage planes. Within the 
same space the quartz is traversed by nine to ten fissures which, although 
not always continuous with the cleavage planes of the schist, yet preserve 
their general direction. All the minute undulations in the schist are gen- 
eralized in the quartz. This is also shown in a specimen from the west side 
of Deer hill. Here there are two undulating quartz laminae generally par- 
allel to each other. While the thicker one makes an S-shaped curve, the 




Fig. 46. — Specimen of schist from locality 550, about J mile soutli of Greylock summit, in natural position, showing in 
upper part a quartz lamina about i incli thick, conforming to the general course of the minute plications, which dips west 
at a low angle while the cleavage dips 60° east. From a photograph. 

thinner one is plicated in the same distance as many as nine or ten times. 
As geologists have observed, such coarse quartz laminae in schist often 
run parallel to the cleavage foliation. In order to arrive at their true strati- 
graphic significance, not only should their general dip over a large surface 
be noted, but allowance should be made for their passing into the cleavage 
foliation for any considerable distance, especially when the dip of that 
foliation forms a considerable angle with that of the stratification foliation. 
Fig. 47 illustrates the relation of quartz lamiufe to the cleavage foliation. 
The cleavage here dips about 50°; the lamijise in a few places, and for short 



148 



GBEEN MOUNTAINS IN MASSACHUSETTS. 



distauees, dij) at the same angle, but vary from 30° to 90°, while their gen- 
eral dip ranges from 40° to 80°; and the stratification dip lies between those 
extremes, being probably higher than the cleavage dip. If it could be shown 

that such laminte are infiltrations in 
fissures following alternately either of 
the foliations, those portions of the lam- 
inae which do not follow the cleavage 
foliation would alone afi^ord reliable in- 
dications, but if their occasional paral- 
lelism to the cleavage foliation repre- 
sents parts of the course of the stratifi- 
cation their general dip should be taken. 
At locality 207, near the junction 
of Grulf brook and Ashford brook, there 
is a large ledge of schist which shows very finely the relations of these pli- 
cated thick quartz liands to both the stratification and cleavage foliations. 
Fig. 48 represents the south side of the ledge. The minute plications 
(stratification) of the schist and of the thin quartz laminae ai-e generalized 




Fig. 47. — Quartz Limiuie in relation to cleavage in 
schist, from locality 12G, south of Deer hill. 




Fig. 48.— South side of schist ledge, locality 207, .junction of Gulf and Ashford brooks, showing the relation of the 
general di|i of the ijuartz lamina- to the minute plications. This dip is 60° to 70°. The cleavage foliation, which includes 
a thick quartz lamina below, dips 35°. Area 14x10 feet. From a photograph. 

in the broader undulations of the thick quartz laminae which have an aver- 
age dip of 60° to 70°. There is also a well-marked cleavage foliation 
dipping 35° in about the same direction. The cleavage planes do not 



MOUNT GREYLOCK. 



149 



traverse tlie thick quartz laminae. Microscopic sections across both schist 
and quartz show the pai'allehsm of the minute phcations of the schist 
with the adjoining quartz, and the cleavage planes of the former terminating 
at the quartz. There is at least one thick quartz lamina in and parallel 
to the cleavage foliation. Through a large part of the more micaceous 
portion of the ledge no stratification foliation is visible to the naked eye 
or under the magnifying glass; and even under the microscope the mass 
shows only a wedge-shaped structure, all the minute folia lying with 
their axial planes either parallel to the cleavage foliation or at a very acute 
angle to it. 




Fig. 49.— Southwest and part of south side of schist ledge (Fig. 48), showing the relation of the two foliations. Area 
15x8 I't. From a }>hotograph. 

Fig. 49 represents the southwest side of the same ledge, together with 
a portion of its southern side, and also shows the relations of the two folia- 
tions. The behavior of the cleavage and stratification foliations, when in 
proximity to a thick quartz lamina, is beautifully shown in Fig. 50, which 
represents a section from a specimen from locality 184, in Goodell hollow. 
The general parallelism of the coarse quartz lamina to the minute plications 
in the schist on either side of it and the cleavage planes arrested by the 
quartz will be observed. The longitudinal cracks in the quartz are pos- 
sibly due to strain, as are also the transverse cracks in the quartz lamina in 
Fig. 40. 

These facts indicate that the dip of the stratification foliation may be 



150 



GREEN MOUNTAINS IN MASSACHUSETTS. 



shown by the general dip of the thick quartz laminae when such lannnte 
can be distinguished from cleavage foliation quartz larainfe. Locality 207 
furthermore shows that stratification foliation may be so completely oblit- 
erated that cleavage foliation alone is determinable. 




Fig. 50.— Thin section of aericite-chlorite-schist traversed by a coarsely plicated quartz lamina, from locality 184, 
Goodell hollow, enlarged 2 diameters, .showing the relation of tlie cleavage to the quartz. lu preparing the slide fractures 
have occurred along cleavage planes. From a photograph. 



CASE VII. 



On the southwest side of Bald mountain, locality 242, the schist is 
traversed by two sets of foliations with different strikes The stratification 
foliation, distinguished by its plications, and in part by the continuity of 
the mineral constituents of the lamina?., strikes north 40° to 50° east, 
and dips 60° southeast. The cleavage foliation strikes north, and dips 
35° to 40° east. The correctness of this observation is coiToborated 
by one at locality 95, on the northern face of Bald mountain, about 
4,000 feet nearly in the direction of the stratification strike as thus deter- 
mined. There the stratification foliation is indicated by great sheets of 
quartz striking north 45° east, and dipping about 75° southeast, corre- 
sponding to the minute plications in the sun-ounding schist, which are 
crossed by a cleavage foliation striking north 3° to 5° east, and dipping 55° 
east. The probable correctness of both these observations is still further 
increased by the trend of the central ridge of Greylock, which, southeast of 



MOUNT GEEYLOCK. 



151 




those localities, is also northeast. A large ledge of schist at Readsboro, 
ill Vermont, in the Grreeii mountain range (Fig. 51), shows on a large scale 
the two sets of foliations and quartz laminse, with diiferent strikes and dips, 
and will serve to illustrate what is not uncommon on Clreylock in similar 
rocks. 

The parallelism between the strike of the cleavage and the strike of 
the axis of the great folds has 
long been recognized in geology 
When, therefore, the axis of the 
fold lies horizontally the strike of 
the sides of the fold will conform 
to the strike of the cleavage ; but 
when the axis of the fold is in- 
clined, i. e., when the fold pitches, 
the strike of the sides of the fold 

Fig. 51. — Sketch of west side of schist ledge in Readsboro 
will not conform to that of the linage, Tt.,showiii{; stratiHc-itiou striking N. 20° E, and dlp- 

l)ing 250 west, crossed liy cleavage striking N. 15° W. and dip- 
cleavasre. This, Prof. Pumpelly I'™S 55° east, with quartz laminjB in both foliations. As the 

'' face of ledge is not parallel with the strike of either foliation 
suggests, is the most probable ex- tlie app-irent angles of dip are not the tme ones. 

planation of these differences between the strikes of the stratification and 
cleavage. The conformity which Heim finds in the Alps between the 
strikes of the two foliations does not hold here.^ 

case; VIII. 

In Goodell hollow, locality 175, southwest of Bald mountain, there is 
a schist with three sets of planes or foliations, set a striking north 5° east, 
and dipping 35° to 45° east; set h striking north 20° east, and dipping 40° 
east; set c striking north 80° east, and dipping 70° north. An enlarged 
thin section (Fig. 52) shows that the minute i^lications follow the direction 
of set b, while set a is formed by a slip cleavage more or less pronounced, 
and set c by the infiltration of dark mineral matter in planes, possibly 
fractures, traversing the other two sets without altering their structure. 
This interpretation of this locality is also confirmed by the strikes and dips 
observed in its vicinity. At locality 132, near the west end and on the 



' See his law 13, op. cit., vol. 2, p. 68. 



152 



GREEN MOUNTAINS IN MASSACHUSETTS. 



south side of the Bald mountain spur, there" is a small ledge in which the 
stratification foliation dips 35° west, the cleavage foliation 25° to 30° east, 
and a secondary cleavage horizontally or very low west. Some vertical 
joints strike north to south through all these planes. 




Fig. 52— Thin section of sericite-clilorite-schist from locality 175, Gootlell hollow, the larger figure enlarged nearly 2 
diameters and the smaller ahout 10 diameters, showing two cleavage foliations crossing the stratification. In preparing the 
slide a fracture has occurred along Cleavage I. From photographs. 

Secondary cleavage foliation occurs here and there in the Greylock 



area.' 



ca-sp: IX. 



Fig. 53 represents an enlarged section of plicated schist from locality 
550, about 1,500 feet south of the top of Greylock. The area in the larger 
and upper fragment measures 1 J by J inches. The specimen from which 
the section was made showed, in its original position in the ledge, a stratifi- 
cation foliation about horizontal or dipping west at very low angle, crossed 
by a cleavage foliation dipping 60° east. From the direction taken by the 
breaks, which occui-red in the preparation of the slide, it seems probable 
that in some portions of the rock here the cleavage foliation dominates. 
Fig. 46 represents a hand specimen from the same ledge in its natural posi- 

' Two sets of cleavage planes are uotlced in the slate on Welden's island, Lake Champlain. Geol. 
Report Vermont, vol. 1, p. 314. A. Baltzer, in the Beitriige zur geologisohen Karte der Schweiz (20te 
Lieferung, Bern, 1880. Atlas, pi. m, fig. 8, and xiii, figs. 14, 16) figures two cleavage foliations 
traversing the same rock. Archibald Geikie, in his report on the recent work of the Geological Survey 
in the northwest highlands of Scotland, describes a double foliation in eruptive gneiss. Quart. Jour. 
Geol. Soc, London, vol. 44, Aug. 1888, p. 398-400. 



MOUNT GREYLOCK. 



153 



tion. The same stratification foliation and cleavage foliation dips recur at 
locality 549, some 2,500 feet south-southeast, and at locality 539 (see Fig. 54) 
about 1,000 feet west, and again at the top of Greylock, and may thus be said 
to characterize the entire eastern portion of the summit of the mountain. If, 
therefore, the larger microscopic specimen in Fig. 53, which only measures 
1^ by J inches, be properly oriented it will correctly represent the structure 
of an area measuring about two-thirds of a square mile, and probably the 
entire east side of the highest syncline of the Gi-eylock mass. (See Sec- 
tions G, H, I, PI. XX.) 




Fig. 53.— Thin section of sericite-iihlorite-schiat from locality 550, about one-quarter mile south of Greylock summit, 
enlarged 2^ diameters, sliowinj; a coarse slip cleavajie crossing a very minutely plicated stratification. In preparing the 
slide fractures occurred mainly in the direction of the cleavage, here the direction of least resistance. From a photograph. 

The microscopic structure thus often epitomizes the general structure 
on one side of a fold. This fact agrees with the drift of what Mr. Heim 
implies in regard to the structure of the Toedi-Windgaellen-Gruppe namely, 
that physical causes have transformed great masses by transforming the 
minute particles which constitute them.' This generalization must not be 



' Op. cit., vol. II, p. 99. 



154 



GKEEN MOUNTAINS IN MASSACHUSETTS. 



carried too far, however, for local changes may occur for a brief space in the 
direction of the plications and of the cleavage foliation, owing to the pres- 
ence of quartz nodules ; or there may also be minor undulations on the side 
of a great fold. 




Fig. 54.— Specimen of schist from locality 539, about one-quarter mile .southwest of Greylock summit, in its natural 
position, facing south. Stratification nearly horizontal ; cleavage dip 500-50° east. From a photograph. 



CASE X. 



Loc.576 



Schist 



The above cases are sufficient to illustrate the structural significance 
of stratification and cleavage and the distinction between them in the region 
under investigation. With the aid of these a fault was detected which 

would otherwise have escaped notice. Near 
the west end of the Bald mountain spur there 
is a somewhat lenticular area of limestone 
trending north and south, and in contact on 
both sides with schist. On the west side the 
contact phenomena are as indicated in Fig. 
55. The limestone overlying the schist dips 
from 45°-60° east, the contact plane between 
both 55° east; the schist cleavage dips 25°- 
55° east, but the plications in the schist dip 
tvest at a somewhat higher angle. The nor- 
mal position of this limestone is under the schist; here it is above in conse- 
quence of a fault. At this point 'the stratification foliation in the schist is 
very much plicated, and the cleavage faulting divides up the rock into lens 




Cleavage 
dip. East. 



Fig. 55. — Diagram showing the relations of 
the Beikshire schist and Stockbridge limestone 
at locality 576, on the Bald Mountain spur, look- 
ing north. The cleavage of the schist conforms 
to the stratification of the limestone, but the 
stratifications are unconformable. 



MOUNT GREYLOGK, 



155 



or wedge-shaped masses. This is the typical slip cleavage. The minute 
structure at the contact, as seen in a microscopic section, corresponds to 
that represented in the diagram. Fig. 55. The inference from such facts 
is that while conformable contacts are all-important in determining strati- 
graphic relations in a metamorphic region they may be entirely misleading 
unless it can be shown that the foliations which conform to the plane of junc- 
tion between both rocks are indeed stratification foliation.' 

CORRELATION OF CLEAVAGE AND STRATIFICATION. 

The facts adduced naturally raise the question as to the general cor- 
relation of cleavage and stratification. The relations of the strikes of the 
two foliations have already been explained under Case vii. As to the dip 




Fig. 56. — Thin Bection of schist from locality 115, on the Bald Mountain spur, enlarged lA diameters, showing the paral- 
lelism hetween the cleavage planes and the axial planes of the plications. From a photograph. 

of the two foliations: The range of the difference in angle of dip between 
cleavage foliation and stratification foliation in sixty-three observations was 
found to be from 10° to 120°;' the average difference 62°, 30'. The abso- 
lute dip of cleavage in ninety-six observations, in which the dip of stratifi- 
cation foliation was also observed, ranged from 10° to 90°, averaging about 
45°; leaving out eleven extreme cases the range was from 25° to 75°, and 
the average 44°.^ The direction of the dip of the primaiy cleavage in one 
hundred and nineteen localities, in which that of the stratification was also 
determined, was distributed as follows: ninety-two localities east or north- 
east, twelve west, four vertical, one south. The southerly dip occurs at the 

• Compare J. D. Dana, Taconic rocks and stratigraphy. Am. Jour. Sci., Ser. ill, vol. 33, May, 1887, 
p. 398, in ■which the possibility of such cases as this is overlooked. 

■^Wben the difference is over 90° the direction of the two dips is opposite. 

'Where cleavage is horizontal and stratification nearly or quite vertical, as is sometimes the case 
in the Berkshire county schist, there have probably been two uplifts. 



156 



GREEN MOUNTAINS IN MASSACHUSETTS. 





Cleavage 
dip East 



W.H.H. 



north foot of Mount Prospect (Saddle mountain) where there is a well marked 
southerly pitch. The westerly dips occur in one of the subordinate schist 
masses, which forms a high cliff west of Cheshire reservoir, and again in a 

low knoll of schist at the extreme sotith limit 
of the map, near Berkshire village, and in a 
similar knoll south of Constitution hill and 
west of Lanesboro. Besides these there is 
one isolated observation between Lanesboro 
and New Ashford, and two others on Ragged 
mountain. So that the observations indicate 
an almost universal easterly cleavage dip on 
Greylock. 

The question may well be asked how 
this can be, since the cleavage is so largely 
associated with the faulting of minute plica- 
tions in strata which sometimes dip east and 
sometimes west. The observations indicate 
that where the sides of a fold dip in a direc- 
tion opposite to that of the cleavage the axial planes of the small plications 
are generally parallel to the cleavage planes, and 
in extreme cases the faulted limbs of the plications 
lie in those planes (see Fig. 56). Fig. 57 represents 
this structure dia grammatically, as drawn in the field. 
Where the cleavage foliation and stratification folia- 
tion both dip in the same direction, but at different 
angles, the structure described in Figs. 56, 57 does 
not occur, and the slip cleavage planes are then 
either parallel with one or with neither of the limbs 
of the plications as in Fig. 59, or else there is a com- 
bination of an extreme form of slip cleavage bor- ,, ^„ T^• ^ . , ^^ 

t: a FiQ. 58.— DL-igram of p,art of north 

derinff on slaty cleavage and of the coarse structure, "If "^ '"'""' 'r'^"' ''"'^"'^' ^"' ""'* 

o J o ^ side ot Deer lull, area 7x5 feet, show- 

described in Case vi, Fig. 48, and seen also in Fig. 58, *"« ™"™'y p""^*"'' i""'^ '^"""* 

*^ o ' traversing the schist, which has a 

in both of which the coarsely plicated quartz laminae ^^^^^^s^ bordering on siaty cleavage. 
are more or less iudepeudent of the cleavage foliation. Or, the cleavage 



Fig. 57.— Diagrams ahowing the relation 
of slip cleavage to stratification at locality 55, 
north side of Mount "Williams, and 862, ridge 
south of Sugarloaf. Cleavage parallel to axial 
planes of plications. 




MOUNT GEEYLOCK. 



157 




foliation, as such, may disappear altogether, becoming merged in the strati- 
fication fohation. Tluis, at the south end of 
Ragged mountain, there is a minor syncHne, 
on the east side of which the cleavage has a 
high easterly dip crossed by plications dipping 
90°, or west, at high angle, while on the west 

. -, p .1 • !• .1 . ,-f i- r 1- • ''"' 5!».— Diagrams showing relation of 

Side 01 this synclme the stratm cation tohation cieavas,. to stratification in scinstwiure both 

T ^»i-n . r>/\(- ^ 1 T ■• . 1 foliationsdipin samedirectiou; cleavagepar- 

dips 25 to 30 east and no distinct cleavage allel to one omeither Umb of plication. 

foliation is visible. (See Fig. 62.) 

PITCH. 

Early in the work my attention was directed by Prof. Pumpelly to 
methods of detecting the pitch of the axes of folds. Observations of pitch 
were made in fifty-four localities on Greylock, East, and Potter mountains. 

In a few places minor pitching folds are 
exposed, as in the limestone at the south 
base of Sugarloaf mountain (Fig. 60). 
But pitch was usually determined by ob- 
serving the pitch of the axes of the plica- 
tions of any part of a fold. The angle 
varies from 5° to 45°, but generally is not 
over 30°. In one or two instances it was over 45°. The correctness of 
the method seems to be verified by the general parallelism which exists 
between the minute and general structure of these rock masses, and also 
by the opposite directi(Mi of the pitch as thus determined, at the extreme 
ends of the mountain.' 

STRrC"rURAL PRINCIPLES. 

From the foregoing data the following structural principles may be 
laid down as applicable primarily to the study of the .metamorphic rocks of 
Mount Greylock, and then to a large part of the Taconic region and to 
similar rocks and regions. 

' See, on the siibjeot. of pitch, Geo. H. Cook, Geology of New Jersey, Newark, N. .!., 1868, p. 5,5; 
on the inclination of the axes of the flexures in the Taconic region, J. D. Dana, Taconic Rocks ami 
Stratigraphy, Part 2, p. 399, already cited* 




Fig. 60. — Minor limestone folds with a northerly 
l)itch, south foot of Sugarluaf, New Ashford. 
Rock 50X30 feet. 



158 GREEN MOUNTAINS IN MASSACHUSETTS. 

Tli« lamination in «cliist or limestone may be either stratification 
foliation or cleavage foliation, or possibly a combination of both. False 
bedding occurs in limestone also. Therefoi'e the conformability of two 
adjacent rocks is only shawn by the conformability of the stratification 
foliation of both. 

Stratification foliation is indicated by : (a) the course of minute plica- 
tions visible to the naked eye, (b) the course of the microscopic plications, 
(c) the general covirse of the quartz laminae whenever they can be clearly 
distinguished from those which lie in the cleavage planes. 

Cleavage foliation may consist of: (a) planes produced by or coincident 
with the faulted limbs of the minute plications, (h) i)lanes of fractm-e re- 
sembling joints on a very minute scale, with or without faulting of the pli- 
cations, (c;) a cleavage approaching "slatj^ cleavage," in which the axes of 
all the particles have assumed either the direction of the cleavage or one 
forming a very acute angle to it, and where stratification foliation is no 
longer visible. These forms may all occur in close proximity. 

A secondary cleavage, resembling a minute jointing, occurs in scattered 
localities, and, although not yet very satisfactorily observed on Greylock, 
original cleavage foliation may become plicated by secondary pressure. 

The degree and direction of the pitch of a fold are often indicated by 
those of the axes of the minor plications on its sides. 

The strike of the stratification foliation and cleavage foliation often 
differ in the same rock, and are then regarded as indicating a pitching fold. 

Such a correspondence exists between the stratification and cleavage 
foliations of the great folds and those of the minute plications that a very 
small specimen, properly oriented, gives, in many cases, the key to the 
structure over a large portion of the side of a fold. 

STEUCTURAL TKANSVEESE SECTIONS. 

On these principles twelve complete and three partial transverse sec- 
tions have been constructed across the Greylock mass; there are also three 
across Stone hill, to which reference will be made in Appendix A. All of 



MOUNT GKEYLOCK. 159 

these are oil the same vertical aud horizoutal scale.' The first section, A, 
crosses the north end of the mass at North Adams; the last, 0, toward its 
south end, between Cheshire and Berkshire villages; and the others at more 
or less regular intervals between. See map (PI. i) for section lines, and 
Pis. xviii-xxii, for sections. 

The sections show that the range consists of a series of more or less 
open or compressed synclines and anticlines, which, beginning near North 
Adams, increase southerly in number and altitude with the increasing width 
and altitude of the schist area, and then, from a point about a mile and a 
half south of tlie summit, begin to widen out and diminish in number aud 
height until they finally pass into a few broad and low undulations west of 
Cheshire.^ Betvv'een that point and the villages of Lanesboro and Berkshire 
the folds become somewhat sharper and more compressed, and the schist 
mass rapidly narrows. The most comprehensive and best substantiated of 
these sections are those two which, beginning near South Adams, cross the 
central ridge north aud south of the summit and then follow the two great 
western spurs aud end near South Williamstown. These sections will now 

be described in detail. 

It 

' Prof. E. Emmons (American Geology, vol. 1, p. 19) gave a section of Greylock running from 
Cheshire harlior, across the summit, and Mount Prospect, to Sweet's Corners and Stone hill. 

Prof. .James Halls section, from Petersburg to Adams, made between 18.39 and 1844, but unpub- 
lished, showed the synclinal structure of Greylock. 

Prof. E. Hitchcock (Vermont Report, vol. 2, pi. 15, fig. 5) gave a section similar to, Imt less 
detailed than that of Emmons. Both of these are drawn on a greatly exaggerated vertical scale, and 
represent the mountain as a simple syncline. 

Prof. J. D. Dana, in his paper on "Taconic Rocks and Stratigraphy" (p. 405), reproduces Emmons's 
aud Hitchcock's sections, and adds several fragmentary ones of his own. On the east side, one west of 
Nortli Adams (Fig. 47), another west of South Adams (Fig. 44); on the west side, one on the west 
flank of Mouut Prospect and north of the Hopper (Fig. 45). and another on the south side of tlie Hop- 
per (Fig. 46); all of whicli simply represent the relations of tlie schist to tlie limestone ou either 
side of the syncline, along the base of the mountain. In his paper on the " Quartzite, Limestone, and 
Associated Rocks of Great Barriugton," etc. (1873, p. 273); and again in his paper "On the Relation 
of the Geology of Vermont to that of Berkshire" (1877, p. 263), he conjectures from the north aud 
south trend of part of the ''llojiper" depression that the Greylock syncline comprises one or more 
subordinate folds. 

- The sections have all been carried down to the top of the ijiiartzite which underlies the Stock- 
bridge limestone. The observed dips have also been indicated on them to enable the reader to dis- 
tinguisli between matter of actual observation and of ordinary induction. The cleavage dips have 
beensimilarly indicated, but ou a separate Hue, and the cleavage foliation has also beeu shown on the 
drawings crossing the stratification wherever both were observed, but it doubtless traverses the 
greater part of the mass. 



160 . GREEN MOUNTAINS IN MASSACHUSETTS. 



TRANSVERSE SECTION G. 



From the Huoaic river at Renfrew milU (South Adams) across Ragged monntain, the central ridge, Symonda 
peak {Mount Prospect), and the north end of Deer hill See PI, xx and Fig. 61. 

Between the most easterly and the most westerly outcrops in the lime- 
stone ai'ea along the east foot of Greylock there is a syncline followed 
westerly by an anticline. This is corroborated by observations about the 
quarries a quarter of a mile north. The well-knt>wn relation of the lime- 
stone to the schists farther up the mountain is not shown here, but may be 
seen on Section B, PI. xviii, about 1 J miles south of North Adams, locality 
28, where the limestone, after fomiing a very small anticline, ruptured and 
partially eroded, dips, a few feet west of it, at an angle of 15° to 30° west, 
conformably under the schist, both rocks striking north 25° east. 

Above, the Hoosic valley limestone comes a mass of schist which 
forms the lower, more precipitous, and wooded slopes, and which, along this 




Fm. 61. — Section G, J'rotii the Hoosic river across Ragged mountain, the Central ridge, Synionds jieali (Mount Prospect) 
and Deer hill. 

section, dips west at an angle of about 30°. Above these schists is a 
bench of arable land stretching for several miles along the east side of 
Ragged mountain. This mountain forms the higher portion of the northern 
end of the range as seen from Hoosac mountain (PI. xii), but is separated 
from the central crest by the "Notch," the south end of whicli is called 
the " Bellowspipe," from the prevalence of wind there. (Bee PI. xvi.) 
This bench on the east of Ragged mountain measures about 600 feet in 
width and is marked by outcrops of a micaceous limestone which here dips 
70° to 75° west. The bench seems to owe its agricultural value in part to 
the rapid decomposition and soil-forming quality of this rock, and })robably 
in part also to the fact that this more deeply eroded strip of the mountain 
flank has formed a receptacle for sand and soil which would have been 
drained off a steep slope. At several points on tlie west side of the bench 
the micaceous limestone comes in close proximity to another mass of schist, 
but the upper contact is covered on this section. At localities 838, 839, SeC' 



U. S. GEOLOGICAL SURVEY 



MONOGRAPH XXIII PLATE XVI 




SOUTHERN END OF RAGGED MOUNTAIN. 
Seen from locality 190, about one-half mile south, showing the easterly dipping Greylock schist (Sg) in contact with the Bellowspipe llnnestone (Sbp) on the west side of Ragged nnountain, and the saddle 14 birdsl due to the erosion of the limestone anticline (Sbpl The hollow to the left is the Bellowspipe. The 

pasture land on the right corresponds to another area of Bellowspipe limestone. From a photograph. 



MOUNT GREYLOCK. 



161 



w 



AibiUc chlar-mtca Schist 



deavoffe tUp 43 °^ 
StraUfdip 



Cleayoffe dip 

^ Sdnv 



tion E, PI. XIX, both rocks dip west, and at 669, Section F, both are Hori- 
zontal, the limestone underlying the schist in all cases. 

In ascending the east side of Ragged mountain, over this second mass 
of schist only westerly dips are met, but on Sections E, C, and again about 
a mile south of Section Gr (localities 204, 126) there are some well-observed 
eastern dips following westerly ones and indicating a syncliiie, which, proba- 
bly being less open at this end of Ragged mountain, escapes observation. 
Near the top is a narrow belt of calcareous schist forming a north to south 
ravine across the ridge and connecting the limestone area of the Notch 
with that on the south. Beyond is a small, isolated schist area which 
forms the south end of the top of the Ragged mountain ridge. The dips 
continue westerl}'. In de- 
scending into the Notch 
the calcareous schist re- 
curs, dipping 60° east and 
indicating another syn- 
cline. The syncliue of 
this small schist area is 
best seen about a half a 
mile south of the section 
line, and has already been 
referred to on p. 157. (See 
Fig. 62.) On the east side of and close to the schist, the calcareous schist 
(plicated) dips 90° and west at high angle; the schist (feldspathic and chlor- 
itic) is also plicated iu the same direction, with a high, easterly cleavage. 
Again, at locality 733, about 500 feet south of the section, the two rocks 
come in contact with westerly stratification foliation and easterly cleavage. 
On the west side of this schist area both rocks are in contact in inverse order, 
dipping east at a low angle. These easterly dipping beds of the west side of 
Ragged mountain stand out in prominent ledges which can be clearly seen 
from the top of a knoll (locality 190) about half a mile south of the Bellows- 
pipe. (See PI. XVI.) The same syncline occurs on Section F and also con- 
tinues south of Section Gr, on the knoll just mentioned, in the limestone and 
calcareous schist area. This limestone is very pyritiferous in places; an 
assay of the pyrite, said to have yielded a small percentage of gold, led re- 




2oort. 



Gz/c.mica-schist ■ 



Fig 62 — Section of sniall syncline at south enil Ragged mountain, showing 
relations of tbe fiiUatinns in the east limb. This section crosses lower part of 
central mass shown in PI. XVI. 



MON XXIII- 



-11 



162 GREEN MOUNTAINS IN MASSACHUSETTS. 

ceutly to some tentative raining here. From the occurrence of the small 
belt of calcareous schist across the top of Ragged mountain and from the 
presence of a well-marked syucline in the western part of the small schist 
area, the structure here has been construed as consisting of two minor folds. 

The section now crosses the " Bellowspipe." Dip observations both 
north and south of the line (see map, PI. i), indicate an anticline here. The 
contact on the west side of the Notch is covered, but along Section F (local- 
ity 709) the micaceous limestone dips west, and the overlying feldspathic 
schists occur a few rods west of it with a similar dip. Some 800 feet south 
of this (Section Gr, locality 589), a quartzite, which frequently replaces or 
is interbedded with these calcareous beds, dips 60° west ; and in ascending 
the hill the nearest outcrop of schist (locality 591), about 500 feet west,' 
also dips west. The relations which occur on the bench on the east side of 
Ragged mountain are thus repeated on the east side of the central ridge. 

The section now crosses the schists of the central ridge about 1 mile 
north of Greylock summit and about a half mile south of Mount Fitch. 
The low westerly dip was observed at several points along the Grey- 
lock road north and south of this section and also at 831 south of Sec- 
tion E. The section then descends into the north fork of the Hopper 
depression. The high westerly dip occurs in the precipitous ravine which, 
beginning about a quarter of a mile north northwest from the sunmiit, finally 
opens into the north fork of the Hopper. Along the 2,100 to 2,200-foot 
contour and extending down to about the 1,900-foot contour, on the west side 
of the central crest and in this north to south portion of the Hopper, is a 
belt of calcareous schist similar in character to that on both sides of Ragged 
mountain, but less calcareous. Farther south, west of Saddle Ball, this 
rock passes into the micaceous limestone. At several points westerly 
dips were found in this belt. It does not recur westward in this portion of 
the Greylock area. From these facts the central crest has been construed 
as a sjmchne of schist with a steep west side, a gently sloping east side, 
underlaid by the limestone and calcareous schist of the Notch and the 
Hopper. 

Mount Prospect (Symonds peak, see PI. xvii), consists of an anti- 
cline, with some minor undulations on the east side and a syncline on 
its west face. This is confirmed by observations on Section E and also 



MOUNT GREYLOCK. 163 

on Bald Mountain, Section I, PI. xx. The presence of the lower limestone on 
the west face of Mount Prospect and of the calcareous schist belt in the 
Hopper, east of it, indicates that its schists correspond to those which, on the 
east side of the range, intervene between the lower limestone (Stockbridge 
limestone) and the calcareous benches. On the west side of Mount Prospect 
(locality 1020), near the contact of the schist with the limestone, there are 
alternations between the two rocks probably due to the erosion of some minor 
folds.^ The contact here with the limestone occurs along the 1,600-foot 
contour, while at the east end of this section it occurs between the 1,200 
and 1,300-foot lines, a fact already noticed by Prof. Dana. 

Between the schist boundary on the west side of Mount Prospect and 
the Hopper brook is an area about a mile wide, in the eastern half of which 
there are numerous outcrops of limestone, but the western half of which 
is covered with di'ift. There is however little doubt, judging from the out- 
crops north and south of the section, that this area is also underlaid by 
limestone, and, if so, that it forms several minor folds. (Compare Section 
I.) It is in the limestone at the foot of Mount Prospect and near the mouth 
of the Hopper that Mr. Walcott observed "several traces of fossils," one of 
which, he says, "appears to be the inner whorl of a gasteropod related to 
Euomplmlus or Madurea? 

Along the Hopper brook, about a quarter of a mile above its junction 
with the Green river, is a small area of quartzite long ago noticed by 
Dewey and Emmons and a,lso referred to by Dana.' In Emmons's section, 

' Such interbedding or minor folding near the line of contact occurs also west of Pittsfleld on 
Hancock mountain, in the Lebanon road. 

^ Chas. D. Walcott : The Taconic system of Emmons, and the use of the name Taconic in geo- 
logic nomenclature. Am. Jour. Sci., ser. iii, vol. 3.5, March, 1888, p. 238. 

' Dewey : " On the stream which issues from the Hopper is areua'cpous quartz of a slaty structure, 
which is an excellent stone for sharpening the chisels used by stonecutters." Am. Jour. Sci., ser. I, 
vol 1, 1819, p. 341. 

Emmons : " The outcrop of the quartz occurs again two miles south, near a mill at the junction 
of the Hopper creek and Green river. A small part only of the mass is exposed, dipping southeast 
and towards the high range of mountains known as Saddle mountains and Greylock." Am. Geology, 
vol. 1, part 2, pp. 12-13. 

Dana: "The quartzite of Stone hill and the quartzitic mica schist of Deer hill in Williams- 
town may be either of the upper or lower quartzite formation, if judged only by the facts the hill 
presents. But the position of these areas, in the Williamstown valley, between high ridges of hy- 
dromica schist, suggests rather that it is the underlying Cambrian." Am. Jour. Sci., ser. Ill, vol. 33, 
May, 1887, p. 410. . 



164 GREEN MOUNTAINS IN MASSACHUSETTS. 

ali'eady referred to, this quartzite, interbedded with mica schist, is repre- 
sented as dipping conformably under the Umestone of the west side of 
Mount Prospect and as separated from the limestone area of the Williams- 
town valley west of it by a fault.^ This he also represents in another sec- 
tion (Greology Second District, p. 145, Fig. 46). The outcrop in the river 
dips about 30° eastwardly, but a few rods southwest up the bank (locality 
11) the quartzite has vertical plications traversed by joints dipping south or 
southwest. Mr. J. E. Wolff finds considerable detrital feldspar in this rock, 
which distinguishes it from the feldspathic schists of Greylock that overlie 
the limestone and ally it to the Stone hill quartzites. Mr. Wolff's report on 
this rock reads as follows : 

"Si)ecinien l()92fl-. Slide: a fine-grained aggregate of quartz and feldspar. 
Stringers of muscovite give to the rock a schistose structure. The feldspars occur 
in irregular, angular giains, part uiistriated, part striated, part microcline. The mica 
and quartz often so surround and cut across these grains as to suggest secondary 
origin of the former. Some of the feldspars contain cores of twinned plagioclase 
feldspar, surrounded by a rim of uutwinned feldspar, or else a core surrounded by a 
rim of feldspar in a different orieutatiou, suggesting perhaps secondary enlargement. 
It seems probable that the feldspar in this and similar rocks is clastic (angular shape, 
different varieties in same rock, etc.) It is noticeable that they do not contain quartz 
and mica belonging to the groundmass, as the pori)hyritic feldspars of the feldspathic 
schists of Greylock often do, suggesting a difl'erence in origin. Tourmaline needles 
occur." ^ 

When in addition to this we take into consideration the fact that 2 
miles south of this l(>cality, on Section 1, there is evidence of faulting, little 
doubt remains that these quartzites correspond to those of Stone and Oak 
hills, and are not to be considered as quartzose beds of the Deer hill schists, 
which are evidently continuous with those on the south side of the Hopper 
and overlie the limestones. 

At the Sweet's Corner dam, about a third of a mile north of Section G, 
the foliation (stratification or cleavage) of the schist strikes north 7° to 12° 

' Emmons: " Along the b se of this mountain [Proapect] is a fracture whose direction is nearly 
north and south, and tlie limestone forming the valley was severed from that of the mountain side by 
an uplifting force." Report on Agriculture, p. 80. See also Geology Second District N. Y., p. 157, and 
E. Hitchcock, Report Oeol. of Vermont, vol. 2, p. 598. 

'^Compare Appendix A, p. 200. 



MOUNT GREYLOCK. 165 

east, and dips SO'^ to 35° east. Immediately east of the Ijridge the land 
rises 40 to 50 feet, forming what is called Sawmill hill. In the schist 
along the foot of this hillock the cleavage strikes north 7° to 10° east, and 
dips 50° to 60° east, but plications are visible here and there, striking east 
or noi-theast, and dipping south or southeast. The same is time of the out- 
crops farther up the hill. These observations are confirmed by those at 
locality 1098, at the north end of Deer hill, along the Green river, where 
the schist plications dip 45° southeast and are crossed by cleavage planes 
dipping 40° east in one place and in another 70° east. On the top of 
the hillock the most northerly outcrop is limestone with somewhat curved 
strata, striking north 5° east, and dipping 35° to 40° east, underlaid 30 feet 
west by schist, with a foliation (cleavage) having a like dip. About 100 
and 140 feet south of this limestone outcrop there are two small masses of the 
same rock with coarse, steep westerly or vertical plications. These may be 
ledges. From all this it has been inferred that the schists of Sawmill hill, 
instead of underlying the limestone as represented in Emmons's section, are 
continuous with those of Deer hill, and overlie the limestone; that the super- 
position of the limestone is the result of an overturn and a fault which have 
caused the schist to dip southeast and the really underlying limestone to 
overlie it with an eastern dip ; and that this fault reappears southward, on 
the east side of Deer hill, where it has brought up the Oak and Stone hill 
quartzites, which underlie the limestone, to the level of the schists which 
overlie it, causing a displacement of about 1,400 feet. 

The section now traverses Deer hill. On the northwest side of the hill, 
at the Grreen river, layers of calcai-eous schist with blue quartz alternate 
with a calcareous, ferruginous quartzite, all dipping 40° east. Their exact 
stratigraphic position is not determinable, but as they are separated from 
the Stone hill quartzites by a considerable area of limestone, as there is no 
evidence of a fault there, and as the schists of Deer hill clearly overlie the 
limestone at localities 7, 8, and 630 on the west, these particular layers have 
been regarded as representing simply a transition from the lower limestone 
to the lower schist. The portion of Deer hill traversed by Section G has 
for these reasons been construed as a syncline, with a fault on its eastern 
side. 



166 



GREEN MOUNTAINS IN MASSACHUSETTS. 



TRANSVERSE SECTIONS H, I. 

From the Hoosic river above MapU drove station {Soulli Adams) across the central ridge, Bald mountam, 
a7id the south end of Deer hill, to the Green river. Also Section H, across the summit (see PI. XX and 
Figs. 63, 64). 

The observations east of the central ridge along this section are few 
and unimportant. The lower schist belt measures about half a mile in 
width, and the area of the overlying limestone and calcareous schist about 
a mile in width. The latter is not overlain here by a subordinate mass of 

schist corresponding to Ragged mountain, but ex- 
tends uninterruptedly, and probably in a series of 
very gentle undulations, up to the base of the cliffs 
which form the east face of Greylock proper. The 
contact between the two rocks, wanting on Section 
I, can be seen in Peck's brook on Section J, the 
Fig. 63.-croaa-a6ction H. calcarcous schist Underlying the feldspathic, non- 
calcareous, micaceous, and chloritic schist, both with a westerly dip. On 
the face of the cliff, locality 549, the cleavage foliation dips 65° east, and 
the stratification foliation 15° to 25° west, and low west or horizontal dips 
prevail to the summit. (See Section H, and Figs. 44, 46, 53, 54.) At the 
top of the ridge which forms the seat of the saddle between Greylock and 
Saddle Ball and so also just west of the Greylock summit the dips are 
high east. The structure of the top of the central ridge here has thus been 
construed as a minor syncline with a steep east slope on the west side and 
a gentle west or horizontal one on the east side. 

The section line now descends a little north of Shattuck flats to the 

I 





Fig. 64.— Croaa-aection I. 



south fork of the Hopper brook. The observations above the flats are not 
conclusive, but in the most southerly ravine, tributary to the south fork of 
the Hopper, westerly dips occur, as they do also in the ravine running north 



MOUNT GREYLOCK. 167 

northwest truiii the summit, which cuts deeply into the central crest, and 
which has already been referred to under Section G. This west dip is also 
shown on Subsection H. The calcareous schist belt is crossed again and 
recurs south in one of the forks of Goodell brook, in both cases witli a 
westerly dip. All this leads to the same interpretation as in Section G, 
excepting that a small anticline seems to intervene here between the sum- 
mit and the calcareous belt, the compressed syncline of the central crest 
having in it a minor fold which does not appear on Section G. 

The section then crosses Bald mountain. Here a great surface of the 
lower schists is exposed. A high northeasterly dip is well determined at 
locality 95 (see Fig. 41), and corroborated at locality 242 on the southwest 
side of the mountain, both with a strike of north 40° east (see Case vii, p. 
150); and an easterly dip recurs high up on the east side of Mount Prospect. 
East of this locality the dip is east in places, but there are probably minor 
folds and much thickening. On Section J, PL xxi, which passes along the 
south side of Bald mountain about 500 feet below its summit, horizontal or 
low west dips occm*, striking with the much steeper dips of the top, and prob- 
ably representing the lower and broader part of the Bald mountain syncline. 
East of this, in the Goodell hollow ravines, there are high westerly dips. 
These facts, and the situation of the calcareous belt in the Hopper, have 
rendered necessary the peculiar construction seen in tlie section. Bald 
mountain thus consists on the east of a sharp anticline turned over to the 
east, followed on the west by a syncline which probably consists of minor 
folds. 

West of Bald mountain, along the spur between the line of the strike 
of locality 242, on the east, and localities 106 and 645 (Hopper), on the 
west, there is an anticline corresponding to the one at the top of Mount 
Prospect followed westerly, between localities 218 and 217, by a syncline 
corresponding to that on the west face of Prospect. West of this again, 
between localities 117 and 217, such a succession of westerly dips occurs 
that it has been necessary to insert a conjectural compressed syncline and 
anticline in order to explain the dips as well as the absence of the lower 
limestone. From the dips in the limestone and schist in the Hopper on the 
northern side of the spur it is probable that another small anticline occurs 



168 GREEN MOUNTAINS IN MASSACHUSETTS. 

between localities 1 15 and 117 on the spur. In fact, judging- from the many 
alternations in the dip and the absence of the lower limestone, the whole 
spur west of Bald mountain seems to consist of a series of minor folds whose 
number probably varies but slightly from that represented in the section. 
Fig. 56, p. 155, represents a specimen from locality 115 on this portion of 
the section. In constructing the section the depth of the limestone has been 
governed by the angle of pitch along the spur and the relations of the Hop- 
per and Mount Prospect (Section G) to Groodell hollow (Section J). 

About three-quarters of a mile east of that arm of the Grreen river 
known as Ashford brook the section crosses a hill known locally as 
" Pine Cobble." On tlie west side of it is a small limestone area cut off by 
schist: on the north, from the Hopper limestone area, on the south from 
the New Ashford limestone area, and on the west from the South Williams- 
town, limestone area. On the east this limestone underlies the Bald mountain 
schists conformably, l)ut on the west side it is unconformably underlaid 
by schist, owing to a fault, the character of which has been partially 
described under Case x, Fig. 55. There would seem to have been a sharp 
ruptured anticline here, the eastern limb of which, consisting of the upper 
400 feet of the lower limestone, with the overlying schist, was thrown up, 
while the western part slid under the limestone, the break having occurred 
along the eastern limb of the anticline in the upper part of the limestone 
bed. This fault strikes with the fault along the eastern side of Deer hill and 
at Sawmill hill, already described, and with the one referred to by Emmons. 
The displacement here can not well be less than 500 to 600 feet. The 
structure of the entire spur also indicates a great deal of compression. ' 

West of the fault the schist dips high west, or 80°, and on the west 
side of Deer hill, a little north of this section, the limestone of the South 
Williamstown A'alley occurs in contact with and under the schist, both 
rocks dipping east. On the east side of Deef hill the dips are 90°, or west, 
indicating a synclinal structure for the central portion of that hill. 

A small ravine skirts the west brow of Deer liill, the east side of which 
is formed by a cliff of schist, the west by a low ridge of limestone. At 

' At locality 331, on west side of Sugarloaf, about 3^ miles south of this part of Section I, there 
is au auticline turned over to the west, bringing the schists under the limestone; and there are some 
indications of a fault between them, but the evidence is not conclusive. 



MOUNT GllEYLOCK. 169 

locality 3'J, a little south of east from South Williamstowu village, the struc- 
ture of the schist is fiuely exposed (see Fig. 58), the coarse stratification 
foliation (plications) dipping about 45° east with a southerly pitch, associ- 
ated with a cleavage foliation dipping 35° east. Following this ravine 
southerly, its sides gradually approach each other until the two rocks are 
finally found in superposition with a westerly dip. 

The chief jioints of interest in the remaining sections will be only 
briefly referred to. 

TRANSVERSE SECTIONS A-F, J-O. 

Section A, PI. xviii, crosses the uortheromost portion of the range at North 
Adams, aud shows a comiiressed syncliue turned over westward.' The actnal contact 
may be seen about a thousand feet west of the North Adams railroad depot, the 
limestone overlying the schist, both rocks striking north 22° east, and dipping 45° 
southeast. I failed witli careful search to find any (juartzite outcrops in this part of 
Greylock, ulthough there are numerous bowlders of it which have probably been 
brought from Clarksburg mountain or beyond.' There is room, between the lowest 
outcrop of quartzite on Clarksburg mountain and the western side of the steep portion 
of the Greylock mass traversed by this section, for a bed of limestone 1,400 feet thick 
dipping at an angle of 50°, which is the dip of the schist at locality fll6 (see map); 
and none of the measurements of the thickness of the lower limestone obtainable on 
Greylock Indicate a greater thi(ikness than that. 

Section B, PL xviii, about a mile and a half south of North Adams. The limestone 
of the Hoosac valley and the schist of Mount Greylock appear here in their normal 
relations. The syncline which farther south consti- ^ 

tutes the central portion of Eagged mountain appears ; 
and there is a second syncline west of it, identical with 
the one on Section A, but open, aud also with that on 
the east side of the N"otch. In the western portion of 
the section two synclines and an anticline have been 
conjectured from observations farther south. It will 
be observed that this section crosses the Greylock ^'"^' 65— Cross-sections a, b. 

mass below the horizon of the upiier limestone and calcareous schist. 

Section C, PI. xviii, about 2 mi es south of North Adams. The calcareous bench 




' See J. D. Dana, Tacouic Rocks and Stratigraphy, Sec. 47, p. 405. Also, On the Quartzite, Lime- 
stone, etc., of Great Harrington, p- 273. 

- J. D. Dana, On the Taconic Rocks aud Stratigraphy, p. 406: ■' A prolongation of it [the Clarks- 
burg mountain quartzite] appears to esteud south of Braytonville into the north end of the Greylock 
mass, along the ascending road (but chiefly on its eastern side) for a mile.'' 



170 GREEN MOUNTAINS IN MASSACHUSETTS. 

on the east side of Rigj,'ed mountain appears with minor undulations. A well-marked 
syncline forms the top of that mountain ; on its west side the calcareous belt is crossed 
twice with an intervening tongue (jf the underlying schist, necessarily anticlinal in 
structure. At the west end of the section there is what might easily be mhtalien for 
imconformnbility between the limestone and schist, a foliation in the limestone (at 
localities 1035, 1036). striking north 77O-80^ east and dipping 250-50° south, while 
the plications in the schist close by and higher up (locality 1038) dip westerly with 
a southerly cleavage, conformable to the foliation in the limestone. It is highly prob- 
able that the fi)liation in the limestone is • cleavage, and that a stratification dipj)ing 
west conformably to the plication in the schist has been obliterated. This would 
make a syncline with the limestone underlying, as in the section. 

Section D, PI. xviii, nearly h:ilf a mile farther south. The Ragged mountain syn- 
cline continues with the upper limestone dipping under it. On the north side of Mount 
Williams there is a bench circling around from " Wilbur's pasture" (the saddle of this 
Saddle mountain), at the south end of the north to south part of the Hopper, and con- 
tinuing into the Notch. The eastern part of this bench is visible from tlie north end of 
Ragged mountain. Along this bench probably passes Formation Sbp — heie, however, 
without any outcrops that are calcareous, exceiJt at 641 and 645 on the north-northwest 
side of Mount Williams. The presence of masses of non-calcareous schist measuring 
from a quarter to three-quarters of a mile in length and several hundred feet wide on 
the northeast side of Ragged mountain and on the southwest side of Saddle Ball in 
the upper limestone and calcareous schist, and the fact that in the Hopper the strata 
of this horizon are much less calcareous and more micaceous than at the south end of 
Saddle Ball or in the Notch, and, finally, the iiresence of noncalcareous quartzite as 
well as limestone in the same horizon in the Notch, all indicate the very changeable 
lithologic character of this horizon. Furthermore, the general synclinal structure of 
the central ridge, the presence of a calcareous belt on both sides of it, and the similar 
constitution of Ragged mountain, together with the fact that at both ends of that 
mountain the calcareous belts are connected, and the greater difficulties involved in 
any other construction of the central crest, all lead to the interpretation given in the 
map, and in this, as well as the other sections. Section D traverses Mount Williams a 
little south of this belt of Formation Sbp. The ba^is for the remaining features of this 
section will be found largely in the next one. 

Section U, PI. xix, crosses Ragged mountain, Mount Williams, and the north end 
of Symond's peak (Prospect mountain). The Ragged mountain syncline passes east of 
the top of that ridge. Along the east base of Mount Williams a long ledge of schist 
shows iilications dipping 40O-15o west, crossed by an easterly cleavage dii^ping 60°. 
These west dips on the east side and the higher westerly dips- on the west side of Mount 
W^illiams indicate the character of the syncline of the central ridge seen farther south 
on Section G. The high westerly dips on the top of Mount Prospect (north end, or 




MOUNT GREYLOCK. 171 

Saddle mouutaiii, Icjcalities G19, 621,) are coustrued as indicating a structure similar to 
that on Section Gr on the same mountain, but more compressed. The presence of an 
area of level arable land measuring about 1,000 feet square — "Wilbur's pasture" — at 
an altitude of 2,200 feet above sea level between the schist masses of Symonds peak 
on the west and of Mount Williams on the east, forjning the saddle of this Saddle 
mountain, and corresponding, as it does, to the similar area, "Shattuck flats," about 
2J miles south, between Bald mountain and the central crest, at an altitude of 2,500 
feet, and also to the calcareous bench on the western and southern side of Saddle 
Ball between the 2,200 and 2,500 feet contours, together with the occurrence of the 
calcareous belt between Wilbur's pasture and Shattuck flats in the Hopper ravines, 
all point to a structural if not to a lithologic similarity. (See PI. xvii.) 

Section F, PI. xix, is confined to Ragged mountain. The syncline and anticline 
observed about the limestone quarries between Zylonite (Howlands) and Renfrew, on 
the mountain side, appear here. The lower schists measure 
only about 1,000 feet on the east side of Ragged mountain 
at this point. In the centre of the Notch, locality 032, 
highly contorted strata of a feldspathic quartzite with a 
low southerly pitch occur. The occurrence of a similar fig. oe.-Crosssection f. 
rock is so frequent in this belt that it may be said in part to characterize the 
horizon.' 

Section J, PI. xxi, south of Section I, from a point a quarter of a mile south of Maple 
Grove station (South Adams), crosses a lens-shaped compressed syncline of the lower 
schist, which is here very graphitic, as it is frequently near the lower limestone. 
At the contact, on the east siile, the schist seems to inclose large lenticular blocks of 
the underlying limestone. West of the main belt of the lower schist is an area, nearly 
2,000 feet wide, of a rock resembling the feldspathic quartzite of the Notch, referred 
to under Section F, but so micaceous as to constitute a fine-grained gneiss.'* The 
strata dip west, and appear to overlie the adjoining schists. For these reasons this 
area has been considered as forming part of the ui)per limestone belt. The observa- 
tions at the west end of this section in Goodell hollow on the south side of Bald moun- 
tain have already been referred to under Section I. Dip observations taken at 
different elevations indicate that the folds become more acute in the lower as well as 
the higher parts of the mass. 

Sections K, L, PL xxi, commence north and south of Cheshire harbor. The schist 
mass east of Cheshire harbor on Section K, which sends out a tongue southwards, 
crossed also by Sections L and M, is that represented in Emmons's section as under- 
lying the Hoosic valley limestone, and corresponding to the schist of Sawmill hi)' 
near Sweet's corners. But observations made by other members of this division of 
the geological survey along the base of Hoosac mountain show that this schist mass 



' Mr. Wolff's determiiiiitions of this rock are given ou p. 185 (locality 345). 
•^ See p. 186, specimen from locality 616. 



172 



GREEN MOUNTAINS IN MASSACHUSETTS. 



probably overlies the Hoosic valley limestone. Along set-tious K and L there is dif- 
ficulty in tracing the connection of the upper calcareous belts of both sides of the 
central ridge, owing to the absence of outcrops on the west side of Saddle Ball. The 
central ridge (Saddle Ball) there slopes ott" to the east at an angle of about 10^, form- 
ing a bench which is even less inclined than that on the west flank of the mountain. 
See the view fi-om Lenox mountain, PI. xv. The conjectural track of Horizon Sbp. 
which on the map joins the outcrop of micaceous limestone at the south end of Sad- 
dle Ball ("Jones's Nose") with those in Peck's brook, Section J, has been drawn to 
conform to the strike and trend of the central ridge, and to those of the calcareous 
belt on its west side. It is based on both structural and topographic considerations. 
(Compare the remarks on Section D.) On the west of the mountain and about Gulf 
brook there are calcareous schists separated from the upper calcareous belt by non- 
calcareous schists. These have been thrown into the lower schists as probably repre- 




FiG. S7.— Cross sections J, K, L. 

senting mere transitions from the lower limestone to the lower schist, such as were ob- 
served at several localities over small areas (Deer hill, 630; Lanesboro, 365; New 
Ashford, 530), and are thus regarded as only indicative of the proximity of the 
lower limestone. 

In Section L the opening out of the compressed and overturned fold of the central 
ridge into a very broad and open syncline is seen. The calcareous belt of the Hopper 
becomes here a gently sloping bench of arable land nearly a quarter of a mile wide, 
once dotted with farms, and still used for pasturage. (See PI. xiv.) The rock 
becomes much more calcareous, aud dips east at a low angle under the upper schists 
of the central ridge, and bends around eastwardly between Saddle Ball and 
Eound rocks, the former consisting of the upper and the latter of the lower schists. 
The upper schists form a cliff on the south side of Saddle Ball at the incision in 
the central ridge, which is seen so plainly from the Tacomic range (PI. xiii), and 



MOUNT GRETLOCK. 



173 



from East mountain (Fig;. 74, p. 194). Here the strata are horizontal or dip very low 

east, and are crossed by a cleavage- foliation, as shown in Fig. 68. The section passes 

along the foot of these cliffs. The upper 

bench of Saddle Ball, shown in Section L in 

the upper schist, and also in the views (PI. 

XIV and Fig. 74), does not correspond to 

any calcareous horizon. A quarter of a mile 

north it measures about 800 feet in width. 

Section L has been extended through East 

mountain, where the strike changes to north 

40° to 50° east, crossing the trend of the 

hill, and a sharp syncline occurs in the schist with the limestone of the Hancock valley 

dipping under it on the west. This schist is continuous with the lower schist of 

the Greylock mass, but the outcrops did not yield further structural data. East 

mountain seems to be one of the subordinate folds of the Greylock synclinorium 

which would thus measure here nearly seven miles in width. 

M 



L,oc.442. .^^ -_^ ^ - — 




2 ft E. 


^^^^^^^a 


^^S^ 


-A^ \\ v'^'A'x ^ 


\\~ 


_<..3«, \ ' Cleayoffe. dzf) -SSE . 


WHH. 



Fig. 68. — Structure in schist in cliffs on south .side 
of Saddle Ball above the Bellowspipe limestone. 




Fia. 60.— Cross-sectious M, N, 0. 

Section 31, PI. xxi, begins about midway between Cheshire and Cheshire Harbor. 
The axis of the central syncline .seems to continue in the lower schists across Round 
I'ocks, where a cliff about 1,000 feet long from east to west and 150 feet high shows 
low east dips at its west end and low west dips at its east end. (See Fig. 74.) 
East of this point observations were few and unsatisfactory. Farther west the sec- 
tion crosses Sugarloaf mountain, which is a small open syncline. (See Appendix 
B.) West of it a number of minor folds produce the frequent alternations of 
schist and limestone about New Ashford. The entire synclinorium here consists of 
a greater number of smaller folds. The section is below the horizon of the upper 
limestone. 



w. 
lotJI5. 




Quart? lamina 
Stratif. 



174 GREEN MOUNTAINS IN MASSACHUSETTS. 

Section N, PI. xxii, begins at Cheshire and shows a synchne in the schist north 
of the Farnham's quarry limestone area. This syncline appears to be continuous with 
that of Sections K and L, and is also on the line of the Bagged mountain syncline. 
North of the Lanesbort) limestone area there are indications of an anticline in the 
schist; and between this and the syncline on the east the numerous easterly dips are 
interpreted as indicating a compressed fold, inclined westward, between the central 
syncline and the eastern one. Between East mountain and the central Greylock ridge, 
in the western part of the section, minor undulations yield alternating areas of schist 
and limestone as on Section M. Both this and the following section indicate an 
increasing compression, the folds becoming more numerous, relatively to the distance, 
less open and more inclined than on Section L. 

Section 0, PI. xxii, starting from Cheshire reservoir, crosses the Farnham's 
quarry limestone area. At the east foot of the high schist ridge, which presents its 

precipitous side to the Hoosic valley (compare PI. xv 
with this section), the limestone evidently dips under 
the schist. At the south end and east side of this ridge 
the schist has a high westerly cleavage, and very low 
westerly or horizontal plications (localities 315, 427, 325J), 
together with a northerly pitch (locality 325). Toward 
the limestone on the west the westerly dip ajipears still 
^ ,„ „. , ,., ^, to continue (localities 325,410,411). The structure at 

Fig. 70. — Structure in schist ou the ^ 77/ 

ridge west of Cheshire reservoir. locality 315 is represented approximately in Fig. 70 ; that 

at locality 325 in Fig. .59, p. 156. 

From the syncline north of the Farnham's quarry limestone area (Section N), from 
the northerly pitch south of it on Savage mountain, from the westerly dip in the 
schist east of that area, and the easterly dip of the same rock west of it (Section O), 
from the character of the dips in the limestone itself, as well as from the isolation of 
this limestone from that of the Hoosic valley, it has been inferred that a schist syn- 
cline underlies the Farnham's quarry limestone, and, therefore, that, although litho- 
logically identical with the lower limestone, it belongs stratigraphically with the up- 
per. We have here, apparently, asmall limestone basin similar in structure and position 
to the larger one which surrounds and underlies Ragged mountain. The difference in 
the limestone of these two areas is mainly in degree of metamorphism. But in several 
places the limestone of Hoosic valley resembles that of the Notch. About half a mile 
SSW of the west end of this section (O), at the east foot of East mountain (locality 749, 
back of Mr. Pine's house), the schist apparently dips east, as does also the lime 
stone. No plications are discernible. If this be the correct dip it indicates an over- 
turn, the dips corresponding to those on the east side of Potter m'ountain (locality 
984) and on the road from Pittslield to Lebanon (locality 1020). 



Cleavage dip 15°w. 



MOUNT GREY LOCK. 



175 



General pitch of the folds. — The observations of pitch are recorded on 
the map by a special symbol. It will be noticed that the direction of the 
pitch through the northern part of the central ridge is south, while at its 
southern extremity, west of Cheshire reservoir, it is north. Sugarloaf 
mountain, New Ashford, has a northerly pitch at its south end, and a south- 
erly pitch at its north end. Ragged mountain has a southerly pitch at its 
north end, and the succession of the horizons at the surface and other facts 
indicate a northerly pitch at its south end. From the "Bellowspipe" the 
pitch is probably both north and south. In places a similar pitch seems to 
prevail along parallel lines across the central ridge as well as the subordi- 
nate folds; thus the southerly pitches on the Bald mountain spur, the north- 
erly pitches on Potter mountain. Constitution hill, and the Noppet, and 
on Savage mountain in Lanesboro; again, the northerly pitch at Cheshire 
Harbor is xuidoubtedly repeated at Round rocks, although not observed there 
in the plications. 

LONGITUDINAL SECTIONS. 

The facts stated above are shown on the four longitudinal sections 
appearing on PI. xxiii. Three of these, on a reduced scale, are given in 
Fig. 71. The north is at the right. 




Fig. 71.— Longitudinal .sections P, Q, R. 

Section P follows for 12 miles the axis of the eastern or Rag-gred 
mountain S5mcline, beginning at the Hoosic river a little south of North 
Adams, between Cross-sections A and B. At the north end of Ragged 
mountain the upper limestone and the upper schist horizons are shown 
with the steep southerly jjitch which marks the whole northern end of the 
Greylock mass (compare the symbols on the map). On Cross-section F 
there is a thinning of the lower schist. There are some indications of a 



176 GREEN MOUNTAINS IN MASSACHUSETTS. 

northerly pitch on the east flank of Ragged mountain west of Rowland's, 
between Sections F and E ; but along the Notch brook the pitch is south 
like that on the Central crest (Section Q). The deeper part of the syn- 
cline is about under the center of Ragged mountaiii. Tlie upper lime- 
stone rises to the surface about a mile soutli of the south end of this moun- 
tain with a gentle northerly pitch, ^ and about IJ miles farther south the 
underlying schists also rise to the surface, forming the pinnacle and the 
neighboring' schist masses which hedge in on the south the northern area 
of the upper limestone. South of this is the Farnham's quarrj' limestone 
area, with well observed opposite pitches north and south of it, forming- a 
shorter and shallower troug-h in the same axis. The section ends on the 
east of Savage mountain. The length of tlie Ragged mountain trough is 
about Zi miles, and the entii-e length of the Farnham's quarry trough, 
extending beyond the limit of the section, would be about 6 miles 

Section Q follows for 14i miles the axis of the central or Clreylock 
syncline, begimaing at the foot of Clarksburg mountain, a little north of 
Cross-section A. From observations made hj other members of this di- 
vision the quartzite of Clarksburg mountain is known to have a southerly 
pitch. The lower limestone is, for topogiviphic reasons, supposed to pass 
completel}^ around between the Clarksljurg and Greylock masses, and thus, 
of course, to conform in pitch to the horizons below and above it. A steep 
southerly pitch is observed at the north end of the central crest, Mount 
Williams. This section shows a deep trough corresponding to that on Sec- 
tion P, but with its center about 2 miles farther soutli, at Cross-section I, in 
the saddle between Greylock and Saddle Ball. The south end or edge of 
this trough is at Round rocks, almost in a line with the south end of the 
great trough in the eastern syncline. This trough is a little longer, meas- 
uring 8J miles. In the incision between Round rocks and Saddle Ball the 
upper limestone and calcareous schists come to the surface. South of this 

' North of thi.s part of the syncline, at the south end of Ragged mountain, tlie vertical distance 
between the top of the upper limestone horizon, where it is overlaid hy the smaller mass of the 
upper schist, and the lowest contour, where the upper limestone occurs, together with the slight 
thickness of the deposit necessitate a southerly pitch. Thus also south of the saddle (the Bellows- 
pipe) ; and for similar reasons a northerly jjitch is supposed between that saddle (Section G) and lo- 
cality 632 in the Notch (Section F). 



MOUNT GREYLOCK. 177 

is a shallower trough analogous aud parallel to the minor one shown oii 
Section P. 

Sections B' and JR" pass through two of the minor synclines on the west 
flank of the Grreylock mass; R' through Stone hill and Deer hill, the syn- 
clinal axis of which probably continues southward through East mountain 
(Section L) and Potter mountain. At the north end (see Appendix A, Stone 
hill) the north pitch is not directly observable, but is pai'tially indicated by 
an observation of Mr. Hobbs in one of the ravines of the Taconic range. 
The relations between Stone and Deer hills are a repetition of those which 
have been inferred between Clarksburg mountain and the Greylock mass, 
the quartzite of Stone hill pitching under the limestone of Green river, and 
that under the schists of Deer hill. 

Section B" passes through Sugarloaf mountain (see Appendix B), one 
of the smaller lateral synclinal axes, which, farther north, appear in Bald 
mountain and Symond's peak. (Sections G and I). In this part of the syn- 
cline, which .measures only about 6 miles in length, there are two well 
marked troughs, one underlying Sugarloaf, and the other the high schist 
ridge south of it. 

RESUME, STRUCTURAL. 

Mount Greylock, with its subordinate ridges, is a synclinorium consisting 
in its broadest portion, of ten or eleven synclines alternating with as many 
anticlines. While the nu nber of these minor synclines is so considerable at 
the surface, it is found, in carrying the sections downwards, that they resolve 
themselves chiefly into two gi'eat synclines with several lateral and smaller 
ones. The larger of these two forms the central ridge of the mass ; the smaller 
one, east of it, forms Ragged mountain and an inner line of foothills farther 
south. The anticline between these coincides with the Bellowspipe; that 
on the west of the central syncline is a little west of the north and south 
part of the Hopper. The major central syncline is so compressed east of 
Syinonds peak (Mount Prospect) and Bald mountain, and its axial plane is 
so inclined to the east that the calcareous strata, which underlie the cen- 
tral ridge, have on its west side a westeiV dijj (Sections G and I). Far- 
ther south this syncline opens out (Section K), and all tlie relations become 

moi-e normal. But between the villages oi Cheshire nnd Lanesboro the 
MON xxui 13 



178 



GREEN MOUNTAINS IN MASSACHUSETTS. 



folds become sharper again and more compressed, and the schist area 
rapidly narrows (Sections N and O), and the structure continues much com- 

f^ pressed to the extremity of the 
mass. On either side of these 

p 

two main synclines the subordi- 

c nate folds are more or less open, 
and have their axial planes ver- 
tical or else inclined east or west. 
The continuity of the folds and 

^ their nuitual relations are shown 
in Fig. 72. Longitudinal sec- 
tions along the two main syncli- 
nal axes (P and Q ) show that the 
trough bottom deepens at two 
points. In the eastern syncline 

H (P) the deeper part of the north- 
ern trough is shown to be about 
under the center of Ragfo-ed 

' mountain, while in the central 
one (Q) it is about 2 miles far- 

, ther south between Grevlock 
and Saddle Ball (Section I); 
and this also would seem to be 

1/ 

the deepest part of the entire 
. synclinorium. The northern 
edge of both of these troughs is 
- M at the exti'eme north end of the 
N Grevlock mass, and their south- 
Q eru edge 7^ to 8i miles distant, 
near Round rocks and the soiith- 
east spur of Saddle Ball. South- 
of these main troughs are two 
shallower parallel ones, the centers of which lie west of Cheshire reservoir 
(P, Q). To the west of these two long axes the mountain mass is made up of 




Flc. 72.— Diagram showing the continuity of the main folds in 
the Grcyhick synelinoriiun. Keducetl from the large sections, 

Pis. XVHI-XXII. 



MOUNT GREYLOCK. 179 

numerous minor folds which do not show the continuity seen in P and Q. It 
will be observed that the direction of these two main synclines represented 
by P and Q is north-northeast to south-southwest, thus nearly parallel with 
the direction of the valley lying between the Clarksburg- granitoid mass 
and Hoosac mountain, and that at the south end they converge, and perhaps 
unite in the narrow schist ridge between Berkshire and Lanesboro vil- 
lages. Traversing the folds of this canoe-like complex synclinorium is a 
cleavage-foliation, sometimes microscopically minute, dipping almost uni- 
formly east. This cleavage-foliation is distinct from the "slaty-cleavage" 
early described by Sedgwick, Sharpe, and Sorby and reproduced experi- 
mentally by Tyndall and Jannetaz, but consists sometimes of a minute, 
abrupt, joint-like fracturing of the stratification laminae, but more gener- 
ally of a faulting of these laminae as the result of their extreme plication — a 
mode of cleavage "Ausweichungsclivage" (slip cleavage) so well described 
by Heim and recently reproduced in part by CadelP by a slight modification 
of the experiments made by Pi'of. Alphonse Favre, of Geneva, in 1878.^ This 
fault-cleavage, when carried to its extreme, results in a form of cleavage 
very nearly approaching, although not identical with, slaty-cleavage. To 
the unaided eye all traces of stratification-foliation are lost, and even under 
the microscope they are so nearly lost as to be of no avail in determining 
the dip. 



LITHOLOGIC STRATIGRAPHY. 

As may be inferred from the descriptions of the sections, there are five 
more or less clearly defined horizons in the Glreylock mass. These are 
described below, beginning with the lowest. 

The Vermont formation.— T\\Q iQ[(\B\)&\\\\(i quartzite of the northwest end 
of Deer hill, which corresponds to the quartzite of Clarksburg and Hoosac 
mountains and of Stone hill, will be noticed more particularly in Appendix 
A, on Stone hill. This is Emmons's "Granular Quartz," and has recently 
been shown to be of Lower Cambrian age. 

The Stockbridge limestone.^The crystalline limestone of the Hoosac and 
Green river valleys, which has long been known to constitute the base of 
Mount Greylock, is the Stockbridge limestone of Emmons, and extends 

' Op. cit. (see p. 137), third series of experiments. 

» Alphonse Fnvre ; The foruiatiou of uioiintains. Nature, \ol. 19, 1878, p, 103, 



180 GKEEN MOUNTAINS IN MASSACHUSETTS. 

tlii'ough Berkshire up into Vermont. It has been shown to be of Cambro- 
Sihirian age. 

The Berkshire schist. — An overlying- mass of schist forms the lower, 
steeper slopes of the mountains on all sides. This is a part of the magne- 
sian or talcose slate of Emmons, Dana's hydro-mica schist, and has come 
to be regarded as of Lower Silurian ag-e. 

The BeUowspipe Umestone. — A series of limestone strata and calcareous 
(sometimes noncalcareous) schists constitutes the higher benches, the Notch, 
and the Farnham's Quarry area. In places the rock is quartzite. This 
horizon seems to have been overlooked by previous geologists on Greylock. 
In 1888 Mount Everett, near Sheffield, in southern Berkshire county; Mount 
, Anthony, near Bennington, Vermont; Mount Equinox, near Manchester, 
Vermont, and Mount Dorset (Eolus), near Dorset, Vermont, were visited by 
the wriier in the hope of finding again on some of these higher summits of 
the Taconic range the upper limestone and calcareous schist of Greylock, 
but a careful exploration of them all failed to yield any trace of this horizon, 
excepting on Mount Anthony. A bench of calcareous schist occurs there 
in the mass of schist above the limestone, but the relations are not suf- 
ficiently clear to enable one to determine whether these calcareous layers 
form part of the Berkshire schist or Bellowspipe limestone formations. Dur- 
ing the year 1889, however, quartzites were found on Monument mountain, 
in southern Berkshii'e, which appear to overlie the Berkshire schist, and 
thus seem to belong to the Bellowspipe limestone formation.^ 

The Grei/Iock schist. — A second series of schists similar to the lower 
ones constitutes all the higher summits of the central ridge and the 
top of Ragged mountain. This forms part of Emmons's magnesian or 
Talcose slate and, together with the Berkshire schist, has been regarded by 
Hall and Walcott as of Hudson River age, and by Dana as representing 
some member of the Lower Silurian. 

All these groups of strata succeed each other conformably. 

• The theory aflvanced by Mr. W. H. Hobbs duriug the printing of this monograph (see Journal 
of Geology, vol 1, Xo. 7, Chicago, October-November, 1893, p. 72.T), that the limestone along the east- 
ern foot of Mount Everett corresponds to the Bellowspipe limestone and the schists which overlie it 
to the Greylock schist recjuires verificeitiott to accord -with lesults farther north. 



MOUNT GREYLOCK. 181 

PETROGRAPHY. 

The petrographic character of the beds of these formations will now be 
described with tlie aid of Mr. J. E. Wolff's notes on the microscopic sections, 
which have been briefly summarized. 

XnE VEUMONT I'OUMATION. 

As the beds of this formation are only represented by one or two out- 
crops in the Grreylock area they will only be described in connection with 
Stone hill in Appendix A. 

tHK STOCKBRIDGE LIMESTONE. 

The lower limestone is a coarsely or finely crystalline limestone or mar- 
ble, usually white, but often banded or mottled, and in places entirely dark 
grey, and there argillaceous. South of and near the South Adams quar- 
ries it is very quartzose, and at the south end of Stone hill there are grad- 
ual passages from limestone to quartzite, the rock consisting of an "aggre- 
gate of calcite grains with rarely a small grain of feldspar and of quartz." 
About Williamstown and along the Grreen river north of Sweet's corners, the 
limestone is very fme grained, and has a hardness intermediate between 
that of quartzite and limestone, and contains occasional quartz grains. 
This fine-grained quartzose limestone may be more characteristic of the 
base of the horizon, but pure quartzite occurs near the top. 

The coarse crystalline limestone is often so micaceous as to resemble a 
gneiss.^ A specimen from a point a little southeast of the North Adams 
reservoir was found to consist of "coarse grains of calcite interbanded with 
muscovite and biotite, and containing occasional porphyritic crystals of 
feldspar. The feldspars contain inclusions of muscovite, rutile, pyrite, etc. 
There are occasional grains of quartz. Some fragments of feldspar are 
microcline, and the calcite cuts across these grains," indicating the possibility 
of replacement by calcite. The limestone about Sugarloaf mountain is also 
quite micaceous. Prof Dewey speaks of the flexibility of this micaceous 
limestone from New Ashford.^ Lenses and seams of quartz are not infre- 
quent. Prof. Emmons noticed the occurrence of albite in the limestone of 

' See E. Hitchcock. Final Report Geology of Massachusetts, p. 569. 

^ "Notice of the flexible or elastic marble of Berkshire county." Am. .Jour. .Sci., 1st ser., vol. 9, 
1825, p. 241. 



182 GEEEN MOUNTAINS IN MASSACHUSETTS. 

Williamstown,^ also the presence of galena and zinc blende here and there 
in small quantities. 

Prof. E. Hitchcock gave five analyses of the limestone of this horizon, 
which show it to be in places a dolomite.^ 

In the upper part, near the overlying schist, occur iiregulai" deposits 
of limonite, as at Cheshire, and along the north side of Mount Prospect, 
and on the east side of Potter mountain.^ Prof Dana has fully explained 
the origin of these ii-ou-ore beds.* 

Towards the upper part of the limestone occur also sti-ata of quartzite; 
thus on the east side of the extreme end of the Greylock schist mass near 
Pittsfield, and also near the Adams quan-ies. 

The fossils foimd by Mr. Walcott, and ah-eady referred to, came from 
this horizon, but fossils seem to be exceedingly rare.^ 

The structural peculiarities of the rock are its almost universal flexure 
into minor pitching folds, and, as already explained (p. 157), its not infrequent 
mini;te plications, and also its cleavage sometimes obliterating all ti-ace of 
stratification. 

THE BERKSHIRE SCHIST. 

This consists of the lower sericite-schists. The groundmass of these 
schists is made up of interlacing fibers of muscovite (sericite) and folia 

' Geology Second District, New York, 1842, p. 158. 

^Fiual Report Geology of Massachusetts, 1841, p. 80, 81. 

'At tlie latter place (Lanesboro Iron company's ore bed) the ore occurs in two positions. In one 
place, owing to an overturn, it lies below the limestone and above the schist. In another it lies on 
the npper side of a small limestone anticline, the schist capping having been eroded. In another 
place a reddish, partially decomposed schist overlies the limestone, the ore probably occurring 
between. The stratigraphic position of the ore is identical in all these cases, however. On the 
Rchist side of the ore there is usually a mass of mottled clay, probably originating in the decomposi- 
tion of the schist, and on the limestone side a yellowish ochre. Manganese ore (pyrolusite) occurs 
here associated with the iron ore (limonite). 

^ Am. Jour. Sci., 3d ser., vol. L4, 1877, p. 132. 

Berkshire geology in ''Four papers of the Berkshire Historical and Scientific Society,'' published 
by the society, Pittsfield, June 1, 1886, p. 19. 

Much of interest in reference to these Silurian limonites will be found also in vol. 15 of the 
Tenth Census (1880), Washington, 1886, especially in the introductory chapter by Prof. Raphael 
Pumpelly on the geographical and geological distribution of the iron ores of the United States (p. 10, 
on the limonites), aud also in Mr. Bayard T. Putnam's notes on the samples of iron ore collected in 
Connecticut and Massachusetts, p, 87. 

■■•Since the completion of the manuscript the writer has found crinoid stems in the upper part of 
the limestone on Quarry hiU, New Ashford. 



MOUNT GKEYLOCg. l83 

of chlorite and grains of quartz. Whether the hydrous character of the 
rock proceeds from the chlorite or from some other hydrous mica can 
hardly be determined, as the two minerals are intimately interlaced. The 
talcose appearance and touch of much of the Greylock schist, which 
have given it the names of talcoid-schist, hydro-mica schist, magnesian slate, 
is due largely to the presence, almost if not quite universal, of these exceed- 
ingly minute folia of chloi'ite;^ and the variable proportions of the chlorite 
and the muscovite in different localities explain the difference in the chem- 
ical analyses of it as well as the variety of names geologists have given it. 
The color of these schists varies with the varying proportions of its prin- 
cipal ingredients — muscovite, chlorite, and quartz. Often it is black from 
the presence of graphite, or porphyritic from the presence of feldspar, or 
spangled from the presence of other minerals. Quartz lenses and seams are 
almost universal. There are also great variations in the texture of these 
rocks. Their structural peculiarities have been described at length on 
pages 138-157, and constitute one of their chief characteristics 

The following is a brief summary of Mr. Wolff's microscopic analy- 
ses of the typical specimens collected : Among the minerals of most fre- 
quent occurrence are black tabular rhomboidal crystals or lenticular plates 
of ilmenite and chlorite, a plate of ilmenite being interleaved between two 
of chlorite. "Similar forms have been described by Renard from the met- 
amorphic rocks of the Ardennes, but they are surrounded by sericite layers 
and not by those of chlorite." He also describes large plates of chlorite 
inclosing small octahedra of magnetite, which also occur on Greylock.^ 
Very minute bluish green crystals resembling the ottrelite of the Rhode 
Island Coal-measui'es are found.' 

Perhaps fully as common, if not more so, is albite, which occurs in 
simple twins or untwinned, sometimes with a rim of clear feldspar separated 
or not from the central crystal by a rim of quartz, and surrounded by 
fibers of muscovite and chlorite. (Thus specimens from locality 458, south 

'See E. Hitchcock, Report Geol. of Vermont, 1861, vol. 1, p. 501. James U. Dana, Am. .Jour. 
Sci., 3d ser., vol. 4, p. 366, and vol. 14, p. 139. 

" See A. Renard, Recherches siir la composition et la structare des phyllades ardennais. Bulletin 
Mus. Roy. Belg., vol. 2, 1883, p. 127-152, and vol. 3, 1885, p. 230-268. 

'See .J. E. Wolff: Ou some occurrences of ottrelite and ilmenite schist in New England. Bull. Mus. 
Comp. Zool., Geological Series, vol. 2, p. 159, 1890. Cambridge, Mass. 



184 GREEN MOUNTAINS IN MASSACHUSETTS. 

of Sugarloaf mountain; 494, between that mountain and Round rocks; 
324, on the line of contact between the Stockbridge hmestone and the small 
mass of the Berkshire schist south of Sugarloaf; 474, in the deep cut 
between east and Potter mountains ; 475, at the southwest end and foot of 
East mountain in Hancock; and 703, at the triangulation point on the north 
summit of East mountain. 

More rarely garnets occur, giving rising to chlorite. Thus at locality 
40, on the tongue of schist north of the Adams quarries. Garnets occur 
also in the small isolated schist mass west of Lanesboro village. 

The graphitic schist of this horizon was early noticed by Emmons^ and 
Hitchcock.'^ It generally occurs near the underlying limestone, as about 
New Ashford, at locality 274, and near Maple Grove station,^ locality 139, 
on the east side of Greylock. The graphite is in microscopic, irregular 
layers, or in masses, surrounded by even sized quartz grains and scales of 
gi-aphite and muscovite. 

Octahedral crystals of magnetite are in many places scattered through 
the schist,^ but the most characteristic minerals are albite, interleaved 
ilmenite and chlorite, and graphite. 

The rock is sometimes calcareous, but not continuously so. Rarely 
veins of calcite and chlorite traverse it. Between New Ashford and Lanes- 
boro a graphitic limestone occurs in the schist, containing angular, often 
rhombohedral, crystals of albite partially replaced by calcite. 

THE BELLOWSPIPE LIMESTONE. 

For structural reasons the Farnham's quarry limestone has been placed 
here. That limestone is generally white (though sometimes gray) and highly 
crystalline, like the Stockbridge limestone; but in the other areas of this 
formation the limestone is finer grained, less often white, frequently argilla- 
ceous, micaceous, or pyi'itiferous. Frequently the micaceous element pre- 
dominates and the rock is a calcareous schist, and in several localities the cal- 
careous element disappears altogether. Galena, zinc blende, and siderite 
occur along with pyrite in the limestone of the Bellowspipe. Associated 
with these limestone and calcareous schists are beds of slightly micaceous 

' Geology of Second District, New York, p. 153. 

' Final Report on the Geology of Massachusetts, p. 581. 

'Emmons, Geol. Second District, New York, p. 141. 



MOUNT GKBYLOCK. 185 

graiiulite or fine grained gneiss, lliese do not seem to be confined to any- 
particular portion of the horizon, nor are they persistent where they do occur. 

The seams and lenses of quartz in the calcareous schist are calcareous, 
and the rock itself is often calcareous where it looks least so, and vice versa. 
In structure it shows the same peculiarities as the limestone and schist of 
the lower horizons. 

No fossils have yet been found in this formation on Greylock, although 
the rock in many places is sufficiently fine grained and not too metamor- 
phic for their preservation. 

The only reason for the entire omission of this horizon from Emmons's 
section seems to be that his section traversed the mountain in one of the few 
places where there are no outcrops on the calcareous belts.^ 

The following is a summary of Mr. Wolff's report on these rocks. A 
bluish gray, finely crystalline limestone composed of calcite grains and 
quartz grains, with occasional flakes of muscovite and considerable pyrite 
scattered tlu"ough the calcite.^ (Thus a specimen from locality 212 on Peck's 
brook, about 2 miles south of the Bellowspipe.) Traversing the limestone 
are thin beds of graphitic, pyritiferous quartzite composed of quartz, feldspar, 
pyrite, graphite, and muscovite. (Thus locality 704 in the Notch about 
three-quarters of a mile south of its highest point.) 

The calcareous schist is composed of large grains of calcite mixed 
with stringers of muscovite and graphite containing inclusions of mica, 
graphite, calcite, and quartz. Pyrite and small fragments of microcline also 
occur in it. (Thus a specimen from locality 712 on the west side of Ragged 
mountain near its south end.) 

The feldspathic quartzite so often associated with or replacing the cal- 
careous schist of this horizon consists of an interlocking aggregate of grains 
of qviartz and feldspar with rare flakes of muscovite, small crystals of rutile, 
and specks of limonite (thus at locality 345 in the Notch, west of the center 
of Ragged mountain); and the gneiss, which seems intimately related to the 
above, is a mixture of quartz with a large amount of feldspar, twinned and 
untwinned j)lagioclase, with occasional grains of microcline and muscovite 

' See Emmons's American Geology, part 2,p. 18. " From the termination of the limestone [i.e., the 
Stockbriilge limestone] to the top of Greylock the talcoso slate is uninterrupted." 

^Recent assays of a similar specimen of this horizon are said to have shown the pyrite to be aurif- 
erous, but not sufficiently so to give the rock any metallurgical value. 



186 GREEN MOUNTAINS IN MASSACHUSETTS. 

plates, magnetite, zircon, rutile, etc. (Thus at locality 616, in the gneiss 
area west of King Cole mountain and Maple Grove station.) 

THE GREYLOCK SCHIST. 

This also consists of schists resembling in their petrographic character, 
appearance, and structure those of the Berkshire schist formation. If there 
be any difference between them it consists in that the upper schists are 
more chloritic and albitic, and less frequently calcareous or plumbaginous 
than the lower ones, but all the minerals occux-ring in the Berkshire schist 
recur in the Greylock schist. 

The interleaved plates of ilmenite and chlorite are the same as in 
the Berkshire schist. (Thus specimens from locality 1,076 in the most 
southerly of the Hopper ravines, about 1,300 feet below Grreylock summit.) 

The magnetite octahedra are also frequently met. (Thus at locality 
449 in the cliffs on the south side of Saddle Ball, and again west of the 
top of Grey lock about a quarter of a mile east of locality 1,076.) 

The feldspathic schists of this formation are characterized here and 
there by large crystals of albite. At locality 709, on the west side of the 
Notch, east of Mount Fitch near section F, the rock might be called an 
albite-gneiss. It consists of "numerous squarish albite crystals, rarely in 
simple twins, crowded closely together," but surrounded by "interlacing 
fibers of muscovite, chlorite, and biotite with magnetite grains and many 
tourmaline needles. Quartz occurs rarely, in little grains or aggregates. 
The biotite and chlorite are often in separate masses, but often pass into 
one another in the same piece. Some of the chlorite may result from the 
hydration of biotite. The feldspars contain inclusions of muscovite, chlo- 
rite, biotite, magnetite, tourmaline, etc." Mr. Wolff separated the feldspar 
of this rock by the use of the Thoulet solution, and a double analysis of it 
was made at the chemical laboratory of the U. S. Geological Survey in 
Washington by Mr. R. B. Riggs (F. W. Clarke, chief chemist). The result 
shows the feldspar to be an almost pure albite. 



MOUNT GREYLOCK. 

Analysis No. 567. Feldspar from specimen 709a, D. I. 1886, 



187 



SiO, 

Al,03+(Fe.03<5%) 

MnO 

CaO 

MgO 

Na^O 

KjO 

Ignition 



I. 


II. 


68.08 


67.83 


20.11 


19.92 


trace 


trace 


trace 


trace 


? 


? 


11.00 


11.65 


.36 


.25 


.31 


.12 


99.86 


99.77 



Dried at 105 C. Sliecific gravity slightly above 2. 6545, between 2. 6.545 ami 2. fil.' 

At the south eiid of the top of Ragged mountain in the small isolated 
schist area (locality 764), the albite gneiss is "coarsely foliated with a 
wavy sti'ucture composed of bands of dark mica, alternating with irregular 
layers of calcite mixed with quartz and large rounded feldspar crystals. 
Needles of tourmaline occur occasionally. The albite crystals are not 
twinned, have a rounded outline, often lie with their longer axes across the 
foliation of the rock, and contain inclusions of calcite, quartz grains, and 
flakes of both micas. The groundmass consists of interlacing fibrous 
layers of muscovite and biotite, little grains of quartz and great quantities 
of calcite, not in grains but in masses. The calcite sometimes penetrates a 
large feldspar, breaking it up into isolated cores of feldspar, surrounded by 
calcite. It is difficult to say with certainty whether the calcite was formed 
later than the quartz and mica or contemporaneously with them. It occurs 
in vein-like masses, not in grains ; when it has encroached on the feldspar 
it does so irregularly and not parallel to the schistosity of the rock, as the 
quartz and mica do; rarely tongues of calcite cut in two inclusions of 
quartz in the feldspars — it seems rather therefore to be pseudomorphous — 
replacing quartz as well as feldspar." 

Towards the top of Greylock and along the central ridge the feldspar 
crystals are very minute and are not rounded. (Thus at locality 861 on 
the Greylock road.) 

' Mr. Wolfi' adds for comparison the analysis of a colorless all)ite from Kiriibinsk, Urals. SiOj 
68.45, AljOa 18.71, FeO 0.27, NaOi! 11.24, K^O 0.6.5, CaO 0.50, MgO 0.18. Total 100. Spec. grav. 2.624. 



188 



GREEN MOUNTAINS IN MASSACHUSETTS. 



Fig. 73 represents a slightly enlarged section of a specimen of the felds- 
pathic schists, which may be regarded as petrographically and structurally 
typical of this formation. 

From all the foregoing the transitional lithologic character of the for- 
mations is manifest.' In the Stockbridge limestone there are passages from 
limestone to qiiartzite and to schist. In the Berkshire schist the rock 
is often calcareous. In the Bellowspipe limestone there are transitions 
from limestone to calcareous schist, and from these to noucalcareous 
schists and to quartzite and gneiss. Mr. AVolff's microscopic examinations 
indicate that this feature is due in part to vainous replacements and other 




Fig. 73.— Thin section of albitic seritite-schist fiom locality 542, between Greylock summit ami SadiUe Ball, 
enlarged IJ diameters. A typical specimen of the Greylock schist, showins the minute plications, the (|uart7, lamina?, 
the slip cleavage with the alhite interspersed. {From a jihotograph.) 

chemical changes at the time of or subsequent to metaraorphi.sm, as well as 
in part to variations in tlie character of the original sediments. 



THICKNESS-. 



The numerous folds, and the fact that they are sometimes compressed 
and overturned, not to mention the difficulties arising from cleavage, render 
exact measurements of thickness very difficult, if not impossible, in the 
Greylock area, but approximations can be obtained. The figures appended 
to the following table are given only as estimates based upon the sections. 
The difference in the estimates arises in pai-t fi-om the varying amount of 
thickening in plication (Stauung). As thickening in consequence of plica- 

' Prof. J D- Daua refers to this in several of his papers on the Taconic rocks. 



MOUNT GREYLOCK. 189 

tion g-enerally occurs in the Greylock mass the actual tliickuess is probably 
less than the minimum figures given in the table, and may possibly be 
considerably less. It will be observed, however, that the maximum thick- 
ness of the entire series does not exceed the minimum thickness attributed 
to the Lower Silurian rocks in the Appalachian region.^ 

GEOLOGIC AGE. 

The question of the age of the beds of Gfreylock, and the treatment of 
the whole subject from the standpoint of historic geology are beyond the 
province of this report, but the various conclusions which have been reached 
and are being reached in regard to the geologic age of these formations are 
added in separate columns for convenience of reference. 



' See J. D. Uana, Manual of Geology, third edition, jiji. Itt2, 210. 



190 



GREEN MOUNTAINS IN MASSACHUSETTS. 



KESUME, LITHOLOGIC STRATIGRAPHY. 

The (jentrul lilholugic character, order, and estimated thickness of the strata of Mount Greylock, East 

mountain, and Stone Mil. 



Formatious, 
natural order. 


/ 




Age.' 


Lithologic character. 


Thickness. 


Emmons, 1855. 


Hall, 

1839-1844. 


Dana, 

1882-1887. 


Walcott, 

1888. 






Feet 










Greylock 


Muscovite (aericite), chlorite, and 


1,500-2,200 


Pre-Potsdam. 


Trenton. 


Lower Si- 


Trenton. 


schist. 


quartz schist, with or without bi- 




Lower Taconic 


(Hudson 


lurian. 


(Hudson 


Sg- 


otite, albite, magnetite, tabular 
crystals or lenticular plates of 
interleaved llmenite and cldurite. 
ottrelite, inicroscopic rutile and 
tourmaline. 
Tliese schists are rarely calcareous 
or graphitic. 




No. 3. "Talcose 
or magnesian 
slate." 


river.) 




river.) 


Bel 1 w s ]> i p ti 


Linieatone, more or less crystalline, 


600-700 


Pre-Potsdam, 


Trenton. 


Lower Si- 


Trenton. 


liiuestone. 


generally micaceous or pyritifer- 




LowerTaconic 


(Hudson 


lurian. 


(Hudson 


Sbp. 


ous, pnssing into a calcareous 
schist, or a feldspathic quartz- 
ite, or a fine-grained gneiss with 
zircon and microcline, or a schist 
like Sb. 
The more common minerals are: 
Graphite, pyrite, albite, and mi- 
croscopic rutile and tourmaline. 
More rare: Galena, zinc blende, 




No. 3. Included in 
"Talcose or mag- 
nesian slate." 


river.) 




river.) 




siderite. 


i 








Berkshiro 


Muscovite (sericite), chlorite, and 


1,000-2,000 


Pre-Potadam. 


Teenton. 


Lower Si- 


Trenton. 


schist. 


quartz schist, with or without 




Lower Taconic 


(Hudson 


lurian. 


(Hudson 


Sb. 


biotite, albite, graphite, magne- 
tite; frequently with tabular 
crystals or lenticular jdates of in- 
terleaved ilmenite and chlorite. 
Garnet, ottrelite. Microscopic ru- 
tile and tourmaline. 
These schists are in places cal- 
careous, especially towards the 
underlying limestone, where they 




No. 3. "Talcose 
or magnesian 
slate." 


river.) 




river.) 
















are often graphitic. 












Stockbridge 


Limestone, crystalline, coarse or 


1,200-1,400 


Pre-Potsdam. 


Lower Si- 


Lower Si- 


Trenton. 


limestone. 


tine; in places a dolomite, some- 




Lower Taconic 


lurian. 


lurian. 


(Trenton.) 


ess. 


times quart zose, or micaceous, 




No. 2. "Stock- 


(Trenton 




Canadian. 




more rarely feldspathic, very rare- 




bridge lime- 
stone.^ 


and lower.) 




(Chazv, 




ly foasiliferous. Galena and zinc 








Califer- 




blende rare. Irregular masses of 










ous.) 




iron ore (limonite) associated 














sometimes with siderite, often 














with manganese ore (pyrolusite). 














Some quartzite. 












Vermont for- 


Quartzite, fine grained, alternating 


800-900 


Pie-Potsdam. 




Cambrian. 


Lower 


mation. 


with a thin- bedded, micaceous, 




Lower Taconic 




(Potsdam.) 


Cambrian. 


ev. 


and feldspatbic quartzite. (The 
latter with calcite, pyrite, tour- 
maline.) Associated with these 
quartzites, and probably at the 
base of this horizim. is a coarse- 
grained micaceous quartzite (tour- 
maline) passing, in places, into a 




No. 1. "Granu- 
lar quartz." 






(Olenel- 
las.) 
















cony:lomerate. and containing 














blue quartz, feldspar (plagioclase, 














microcline) and zircon, all of 














clastic origin. 














Total thickness: 














Minimum 


5,000 
7,200 












Maximum 











' For Prof. E. Emmons's views see his works already referred to, especially his American Geology, Part 2, pp. 10-18, 
48, 128. 

For Prof. James Hall's views, announced as early as 1839-1844, but not then published, see American Journal of 
Science. 3d ser., vol. 28, October, 1884. p. 311 : " Prof. James Hall on the Hudson river age of the Taconic slates." Also 
Jules Maroon : "On two plates of stratigraphical sections of Taconic ranges by Prof. James Hall," Science, vol. 7, 1886, 
p. 393. New York. 

For Prof. Dana's views see his papers: " (xeological Age of the Taconic System," Quarterly Journal of the Geolog- 
ical Sociely of L(mdon, vol. 38, 188"J. p. 3'.)7 ; "On Taconic Rocks and Stratigraphy," American Journal of Science. 3d ser., 
vol. 33, May, 1887, p. 410. and also 'On the Hudson river Age of the Taconic Sc-hists," etc., ibid, vol 17, 1879. p. 375. 

For Mr. diaries D. Walcott's views see the map and section appended to bis paper, " The Taconic system of Emmons, 
and the use uf the name Taconic in geologic nomenclature," American Journal of Science. 3fl ser., vol. 25. April, Ma.y, 
1888, pp. 307, 394, pi. 3, also "The Stratigraphicai auccessioa of the Cambrian Faunas iii North America" (abstract of hie 



MOUNT GREYLOCK. 191 

AREAL AND STRUCTURAL. 

The geologic map of the Greylock, East and Potter mountain masses, 
presents a great body of" the schists of the Berkshire schist formation, sur- 
rounded by the underlying Stockbridge limestone. It is probable, although 
not demonstrable, that this limestone passes around the north end of the 
Greylock mass, between the schist on the south and the quartzite (Vermont 
formation) of Clarksburg mountain on the north. It is also probable that 
that quartzite underlies the entire Greylock synclinorium, for it occurs on 
the north on Clarksburg- mountain, on the east on Hoosac mountain, and 
on the west on Stone liill, and is also brought up again by a fault on the 
east side of Deer hill. 

The Bei'kshire schist sends out tongues corresponding structurally to 
syuclines into the lower limestone area, as west of Zylonite on the east side 
of the range, and at Deer hill on the west side ; also at Constitution hill, 
west of Lanesboro. There are also reentering angles of limestone in the 
schist area, corresponding to anticlines, as nortli of Lanesboro, and about 
New Ashford. 

There are isolated schist areas, generally lenticular in form, corre- 
sponding to more or less open synclines, as a little southwest of South 
Adams, and south of Constitution hill, in Lanesboro and about New Ash- 
ford. The most interesting of these is Sugarloaf mountain, which is a 
canoe-shaped open syncline. (See Fig. 74, Appendix B, and Sections 
M, R.") 

There are also isolated limestone areas, corresponding to compressed 
anticlines, projecting through the overlying schists, exposed by their erosion. 
Two of these occur between New Ashford and Lanesboro, and a smaller 
one is described in Appendix B, at Quarry hill, New Ashford. (Figs. 

remarks before tho lutornational Geological Cougress. Lourtou, September, 1888). in Natiu'e, vol. 38, No. 23, October 4, 
1888, p. .551; also Ills paper, "Straligrapbic Position of tbe Olenellus Fauna in North America and Europe," American 
Journal of Science. 3(1 ser., vol. 37. May, 1889, p. 374. 

For a defense of Emiuou.s's classilicatiou see "Pal:i?ontologic and Stratigraphic Principles of tbe adversaries of the 
Taconic," by Jules Marcou, American Geologist, July, 1888; and for Mr. Marcou's own classification of the Taconic 
rocks see his memoir, "The Taconic system and its position in stratigraphic geology,"' Proceedings of tho American 
Academy of Arts and Sciences, vol. 12. Cambridge, ISi^-S, p. 174. 

For a summary of the dilfercnt phases of opinion in regard to the age of the Taconic rocks see "A brief history of 
Taconic itleas,"' by J. D. Dana, Am. Jour. Sci., 3d ser., vol, 36. December, 1888, p. 40. 

For tbe literature and a 8y.=demati<-. presentation of the Taconic question .lee Bulletin 81, U. S. Geol. Survey, Correla- 
tion Papers — Cambrian, by Chas". D. Walcott. For evidence of the Loweri Cambrian age of the lower part of the Stockbridge 
limestone, see article by J. E. Welti", "' On tlje Lower Cambrian age of the Stockbridgelimestone," Bull. Geol. Soc. Am., vol. 
2, 1890, p. 331. Also paper by T. Nelson Dale, " On the structure and age of the btookbridge limestone in the "Vermont 
Talley,''Bull. Geol, Soc. Am., vol. 3, 1891, p. 514, 



192 GEEEN MOUNTAINS IN MASSACHUSETTS. 

78, 79.) The limestone area iu the western part of the Bald mountain 
spur is anticlinal in structure, but faulted. 

The relations which have been described above as existing between 
the lower limestone (Stockbridge limestone) and the lower schist (Berk- 
shire schist) are repeated at a higher level between the upper limestone 
(Bellowspipe limestone) and the upper schist (Greylock schist). Ragged 
mountain and the higher portions of the central ridge (Saddle Ball, Grey- 
lock, Fitch, Williams) are synclinoria of the upper schist resting upon and 
surrounded by the upper limestone. The tongues and reentering angles and 
isolated schist areas occur here, as well as in the lower formations. But the 
isolated limestone area southwest of Cheshire, instead of being an anticline 
of the Stockbridge limestone projecting through the Berkshire schist, seems 
to be a syncline of the Bellowspipe limestone resting upon the Berkshire 
schist, homologous to that which encircles and underlies Ragged mountain, 
but without any similar mass of schist on it. The relative height of the 
surface of the Farnham's quarry limestone, as shown in Section P, accords 
well with this interpretation.^ 

RELATIONS OF GEOLOGY TO TOPOGRAPHY. 

It remains now to show the relations of the structural, lithologic, and 
areal geology to the surface features. We fhid here evidence of the opera- 
tion of several causes : 

First. The mineralogic character of the rock, presenting minerals more 
or less easily disintegrated by physical or chemical agencies. 

Second. The internal structure and position of the strata, forming ele- 
vations and depressions in the mass and determining the surface relations 
of the different kinds of rocks. 

Third. Erosion, glacial, as well as pre-glacial and post-glacial, bringing 
physical and chemical agencies to bear upon those irregularities in the form 
and composition of the surface. 

' These facts are brought out on the accompanying map and the sections (Pis. i, xvm-xxili). 
Besides the usual dip and strike symbols there have been added on the map symbols indicating the 
direction and angle of jiitch, and also the symbols proposed by Dr. H. Reusch (op. cit.) to indicate 
the cleavage dip and strike, aud IJually, uumbers of the important localities referred to in ttis 
report. 



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55 '£ SQE. 
I RAGGED W (RAVENS CRAG) 

ap'-^o'w -is'w w. sq- so'60''iv 7S°iv 

ifiO'tV. 60£ W SS'-?d' WWW. W 90' 



SO-SS'W. 30' 

45 "f 
. 60 £ 




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RENfREW MILLS 



+ 'Ks 



Srr/fon Ct . 



MT. (JHKVliOC Iv 

Verlicul MUorizoiilnl ScalcinioS orl-i inrh('s=l iiiiU'. 

SyMBOLS 
CHLOR-MICA ISCRICIT£)SCHIST 

CALCAREOUS MICA-SCHIST & LIMESTONE 

in plactsilte Schist is norCalcareous, in places QuBririte 

LIMESTONE 

OUARTZlTE- 




Odscrverl 



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hiif tf frnvrr'.scft fhr i/rcrth-/' pfirf o/'thf nurss 






Sca//i 



^EA LEVEL 



.3500 FT. 
3000 
.2500 
,2000 
. ISOO 
. /poo 
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.SOO 

. 1000 



HOPPER BROOK 

SOUTH FORK 



E SS°C 60' E 

M^GREYLOCK 

; SUMMIT 3S0S r-' (LONG IS'IO') 
VJ W. W 60-7O'W.'0'tlrV/ \ 0'W3Si40^W- 

I , I TOjTS't. 'f.OW W 




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(Jhscx'Vft/ 






40-4S'E. 

, SO-SS'E 



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SEA LEVEL 



300O FT. 
3S00 
2000 
-1600 
1000 
SOO 



GREEN R 'W. WHIGH 
WILLIAMSTOWN 30'. 
& NEW ASHFORDBOftO '.90° 



£ E 40'E >V oO'C SO'W 



HOOPER BROOK 
SOUTH FORK 



35'E 60:E. S0-SS''£. 

'central crest 
e high 0° 6' long ts'io'. 



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SecWon I. 



us GEOLOGICAL SURVEY 



MONOGRAPH XXin PLXXI 



MT. (;nKVF.()('K 

Verliral Fi-Horizv)iil;iI ScnlciTHSB oris iiulic.s=l niilc 

SYMBOLS 
CHLOR-MIC^ IS€RICITEISCHIST ._ . 



CALCAREOUS MICA-SCHIST & UMESTONE 

in places the Scfiisris notCalcareous. m pieces (fuartzite 



LIMESTONE . 
qUARTZITE. 




Oh.svn Tf/ I 

_ 3200 f' 

. 2500 
. 2000 



-. 1000 

soo 

^SEA LEVEL 



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Mr n. 



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ROAD B/t00H3!,'C 



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60-£. SOe 55 £ 
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30' IS-25' 20 E O'O" 

E E E LOW lowC £ 



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£ BASSEn BROOK 




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40'SO'IS' 45"'. 60"40^ SUGAR w 

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"ROUNO ROC Kfi" 

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SC SPUR or S0'30'<o'LOHG 

SAOOLEaALL £. £ 73'I0' 




U. 8. GEOLOGICAL SUHVEY 



V SotXifmsf tjUurt "/"/ 



M^^GREYLOCK 

Longitudinal Sections. 



P THE EASTERN SyNCLINAU 



L Syrnbnlj aj en tratwvmtf vrCwna I 



MONOflftAPM XXHI PLATE * 



tioosic tiivtn 




THC CENTffAL S'KNClINAL 







TYiansi'cr.'tt- StMf^nns. 1)1 




GREYLOCK LONOITUOINAL SECTIONS, P. Q. R. 



MOUNT GEEYLOOK. 193 

The interaction of all these have molded the mountain and given it 
its varied topography. 

The phj^sically and chemically more resistant schists form the more 
elevated portions; also the steeper and more rugged and wDoded slopes, while 
the broad, cultivated valleys of the Hoosic, Green, and Housatonic rivers, 
and the more gently undulating portions of the mountain generally correspond 
to limestone areas. The upper limestone strata and calcareous schists con- 
stitute the benches of agricultural and pasture land, which form so marked a 
feature in the Greylock landscape, and to which attention was directed in the 
Introduction. Thus the Notch and the agricultural character of its surface 
find their explanation partly in its anticlinal structure and partly in the cal- 
careous element of its strata. The character of the bench on the east flank 
of Ragged mountain has already been noticed. Similarly the broad bench, 
which extends for 2 miles at an altitude of 2,000 to 2,500 feet above sea 
level on the west side of the central ridge between Gt-reylock and Saddle 
Ball and around "Jones's Nose" (see Pis. xiii, xiv), corresponds to the 
gently inclined strata of Formation Sbp, witli its easily weathering and 
subsoil-forming micaceous limestone. This accounts for the farms which 
once dotted its surface, still mostly recognizable as open pasture land. 
Thus, also, is explained the incision between Round rocks and Saddle Ball. 
(Fig. 74 and Section Q.) 

The 2^-mile long north to south extension of the great Hopper cut was 
partly occasioned by the trend of the folds and partly by the upturned edges 
of the calcareous belt, which, on the north, at "Wilbur's pasture," and, on 
the south, at "Shattuck's flats," still retain something of their former surface 
outline. (See PI. xvii.) Prof Dana's surmise that the north to south Hopper 
depression is due to a subordinate anticline^ is correct, but the anticline seems 
to occur on the west side of the Hopper. The main east to west Hopper 
incision does not seem to correspond to any structural featm-e, but to be 
simply the result of the surface drainage of the west slope of the range eating- 
back, i. e., eastward, through the subordinate folds until it reached the cal- 
careous belt, and then, owing possibly in part to the sharpness and conse- 
quent weakness of the anticline west of it, but mainly to the more assailable 

f 

1 On the quartzite, limestone, etc., in the vicinity of Great Barrington, Massachusetts, p. 273. 
MON XXIII 13 



194 



GREEN MOUNTAINS IN MASSACHUSETTS. 



character of the calcareous belt, and its general trend, erosion proceeded 
quite as rapidly laterally, north and south, along the strike as easterly across it. 
The deep incisions south of the Bald mountain spur (Goodell brook, 
Mitchell brook, etc.). and the corresponding ravines on the east slope of the 
range (Peck's brook, Bassett's brook, etc. ) are the usual effects of the drain- 
age of a mountain range ; and the alternation of precipice and gentle declivity 
in these ravines is explained by differences in the character of the noncal- 
careous schists themselves, and also by the alternation of calcareous and 



Sadd/e 03//. Jones'A/ose NewAs/rfordCfy. Rouncf ffocks. SugsrLoaf 
UpperSc/t/sf: Ca/c.Schisf- Lower Sc/tisf 



s,';^' 



N. 




Fig. 74.— Outline skftili of Rcmiirt rocks aud the northern alope of Saddle Ball and Sugarloaf mountain from th- 
west, locality 772 on East mountain, ahowinp; the hollow between Round rocks and Saddle Ball due to the erosion of the 
calcareous schist (Bellowspipe limestone) ; also the cliff at Round rocks in the Berkshire schist, and the upper bench ou 
Saddle Ball in the Greylock schist. 

noncalcareous schists. Some (.)f these ravines are quite as steep aud diffi- 
cult of access as any in the Hopper. 

The problematic upi)er bench on Saddle Ball (see PI. xiii, Fig. 74 and 
Section K) is possibly due in part to the horizontal position of the strata 
along a portion of the slope, aud possibly in part also to pre-glacial erosion. 
These benches and tliose on the long southeast spur of the same mountain 
may also be coimected with the gentle northerly pitch of the south end of 
the great troughs. They require further study. 

The saddle between Greylock summit and Saddle Ball seen for a great 



MOUNT. GREY LOCK. 



195 



distance south (PI. xv) is due to the syndinal structure of the central ridge, 
the westerly dip onlhe east side of Grreylock, the easterly dip on the west 
side of Saddle Ball. The saddle in the central crest as seen from Mount 
Equinox, i. e., the north northwest, is due to the pitch of the sides of the 
central trough. (See Fig. 30 and Section Q.) The northeast to southwest 
trend of the ridge between the two summits and the northerly trend of the 
central ridge north of Grreylock correspond to changes in the direction of 
the strike, but the general trend of East mountain does not conform to the 
strike of its strata. 

The two depressions, alternating with three elevations, seen on the range 



Alt Prospect- BcddMt ^ur 



•ScuUILp- Ball. 



I~t/>iLn£L.fiac/ts 




Fig. 75.— Sketch of the Greylock m,iss from the .southwest (locality 1008, on north Potter mount.iiii) showing the .surfiice 
of the Bald nimmtain spur anil of Round rock.s pitching toward each other owing to the pitch of the .syurlinoi-ial axis. 

from Clarksburg mountain and the Stamford valley are due to the presence 
of the two iDelts of the upper limestone and calcareous schist on either side 
of the central ridge (Berkshire schist) one forming the Notch, the other 
"Wilbur's pasture," and the north to south part of the Hopper. 

The gentle northerly slope of the surface from Hound rocks to Jones's 
Nose (see Fig. 74, and Section Q), and the similar southerly slope of the 
top of the Bald mountain spur, as seen from North Potter mountain on the 
southwest (Fig. 75), are probably due to the trough structure of the entire 
mass, the former constituting a part of the northern trough of the great cen- 
tral syncline. To this structure are probably also due the long, steep south- 



196 GREEN MOUNTAINS IN MASSACHUSETTS. 

ern face of Round rocks and the steep south side of Saddle Ball. The former 
is a very striking object in the landscape both from the east and west. 
(Compare Section Q with PL xiii and Fig. 74). An east to west system of 
joints and fractures growing out of the pitch may have aided glacial and 
other erosion at these points. 

The great west spurs which characterize the west side of the range 
(PI. xiii) are portions of the mass left by the erosion which chiseled out the 
Hopper and the hollows farther south, while the pleasing variety of surface 
features seen on the east side from Hoosac mountain (PI. xii) is the result 
of the Berkshire schist forming a sei'ies of foothills between the upper and 
the lower limestone.. Some of these are also shown in PI. xv, the view 
from Lenox mountain. East of the summit, however, these schists have been 
eroded almost down to the level of the Stockbridge limestone, thus enabling 
one to look over from Hoosac mountain into the area of the Bellowspipe 
limestone and southwards for 2 miles to a point where the Berkshire schist 
rises from under the Bellowspipe limestone and hedges it in ("The Canoe" 
PI. xii), forming several considerable masses, the pinnacle and the southeast 
spur of Bald mountain. These constitute the ridge between the northern 
and the southern trough of the eastern syncline and shut in the view. (Com- 
pare PI. XII and Section P.) 

A careful comparison of the topography and geology of the map, with 
the transverse and longitudinal sections, and the general views (Pis. xii, 
XIII, XV, and Figs. 30, 75) will show more clearly than words can the general 
structural relations of the Grreylock mass to its surface features. 



APPENDIX A. 

STONE HILL, NEAR WILLIAMSTOWN. 

This oft-studied and problematic locality has not yielded anything very remark- 
able.' Observations of strike and dip were made, typical rock specimens were col- 
lected and submitted to Mr. Wolff for microscopic examination. Three cross sections 
have been constructed, S, T, U (Fig. 76), and one longitudinal one, E' (PL xxiii). The 
dififlculties at Stone hill arise from the small number of outcrops and their entire 
absence at critical points. 

The areal geology of the hill is indicated on the geologic map. The first question 
which arises is whether the mass of quartzite along the east side of West brook val- 
ley, apparently overlying the .limestone, forms a part of the quartzite at the top of 
the hill. There is a gentle slope of arable land between the two, and a small lime- 
stone outcrop on the east side, at the north end of the westerly mass, has a foliation 
which strikes with the trend of this strip of cultivated land. It has therefore been 
conjectured that the two masses are separated by limestone, but the other supposition 
would be tenable. 

The dips in the main mass of quartzite, on both sides and in the center, are 
easterly; but at the south end the dip (pitch?) is south, and a well marked southerly 
pitch occurs in the quartzose limestone at the southwest end of the hill (localities 
1103-1105). A very high southerly pitch occurs also iu the limestone a little farther 
south (locality 62) on the north side of the Green river bridge crossed by the road from 
Sweefs corners to South Williamstown. Here there is a small, sharp anticline with 
an almost vertical pitch. A southerly pitch occurs again iu the schists at the 
north end of Deer hill. High up on the southeast side of Stone hill (locality 1106) 
an outcrop of quartzite with limestone north of it shows a southeast pitch. This, 
however, has been regarded as a quartzose part of the limestone. Formation €3Ss. From 
all these facts the quartzite at the top of the south end of Stone hill appears to pitch 
under the limestone farther south and down the hill, and that limestone to i>itch 
under the Berkshire schist of Deer hill. We thus have here in their normal succes- 
sion, the Vermont formation, the Stockbridge limestone, and the Berkshire schist, 
and the relations which seem to exist between Clarksburg mountain (Oak hill) and 
the north end of the Greylock mass are repeated here between Stone hill and Deer 

' See Emmons: Geology of the Second District of New York, pp. 145, 156, 159; Report on agricul- 
ture pp. 83-86. James D. Dana: Taconic rocks and stratigraphy, p. 406; Geology of Vermont and 

Berkshire, p. 206. 

197 



198 



GEEEN MOUNTAINS IN MASSACHUSETTS. 



bill. (See sections Q and R')- These relations at the south end of the hill, together 
with the structure of Buxton hill and the northerly pitch observed by Mi-. Hobbs at 
locality 2005, a little west of Buxton hill, lead to the supposition that the quartzite 
of the top of the hill, at the north end, pitches under the limestone at Williamstowu. 
The correctness of this conclusion is also rendered probable by the petrographic 
character of the Stone hill beds, which is similar to that of the Oak hill beds. On 
the east of Stone hill strata of micaceous feldspathic quartzite occur between those 
of massive quartzite (locality 627). In three localities a flue schist or phyllite of 



Observed 
Stratify dips 



. lOOOFl 

. 700 
.500 



TooLof TaconicRanae roaa 

l5'-2Q3tf 5S6OWMt,S0° 50" North part of 45° Probable Green 

E.El E.prookE E. Stone Mill t. Fault R. 



_Sea level 



Izl888 




660 Ffc 



Observed 
StratiFC dips 

EioooFd. 
700 
500 
300 
Sea level 



West 
Brpoh 



4d'S0°3Z42°65° Probable 
roadEX. f. E- Fault, 



-*-E 

Green 
R. 




•CSS. 700 Ft, 



Observed 
StratiFC dips 



i 



ioooFl 



700 
500 



West 
Brook 

I 

I 



V >E 

Stone Hrll 75^55?60' Green 

roao South part LE. PtDbatJieFault. R 




720 Fb. 



•C55 

Sea level "df:'') 

Fig. 76.— Cross-sectious S, T, U, Stone Hill. 

inconsiderable thickness appears. Towards the north end of the east side of the 
hill a blue ijuartz conglomerate, and a quartzite containing blue quartz and detrital 
feldspar occur.' Mr. Wolff's descriptions of these rocks are given beyond. 



' Dewey : "Granitell of Kirwan, quartz, and feldspar. This aggregate forms extensive strata at 
the east base of Stone hill, the feldspar is ditt'iised in grains through the quartz, and sometimes crys- 
talline, forming porphyritic fjuartz. This aggregate is often compact and very hard, hut frequently 
it is porous and hard, forming good millstones. Sometimes the quartz appears in such fragments 
that the stone resembles breccia." Am. Journal of Science, ser. i, vol. I, 1819, p. 343. 

See also Emmons, American Geology, p. 16, on the conglomerate of the granular quartz at Oak hill. 



STONE HILL. " ]iJ9 

In constructing transverse sections of Stone hill several difficulties present them- 
selves. The quartzite with detrital blue quartz and feldspar, which may naturally 
be supposed to occur near the base of the quartzite and towards some underlying 
gneissoid rock, and which Emmons places at the base of his "granular quartz," 
occurs only on the east side of the hill dipping toward the limestone outcrops of 
Green river and Williainstovvn (Formation €!Ss). On the west side of the western 
mass of quartzite the rock is massive, and seems to be conformably underlain by the 
limestone of Formation €Ss, but that quartzite we should expect to reiiresent the 
upper part of the quartzite (Formation €v). 

One explanation of these facts would be that on the west the apparent superpo- 
sition of the quartzite upou the limestone is the result of an overturn, while on the 
east the two rocks are separated, as Emmons supposed, by a fault.' Such a fault 
would be nearly, if not quite, on the line of the fault on the east side of Deer hill (Sec- 
tion G), and with that farther south near the west end of the Bald mountain spur 
(Section I) and also on a line with faults in southern Vermont at East Pownal. The 
highly contorted character of the limestone strata along Green river east of Stone 
hill, and in the village of Williamstown^ also leiul probability to such a hypothesis. 

Upou this basis of fact and probability the folds in the Stone hill sections have 
been constructed. On the east side of Stone hill a fault is represented; the central 
portion of the hill consists of a syncline followed on tlie west by an anticline over- 
turned to the west; the outlying masses of quartzite on the southeast and northwest 
sides of the hill involve two minor anticlines. All the folds have a southerly pitch at 
the south end of the hill and a northerly one at the north end. 

The entire thickness of the Stone hill (juartzite and its associated micaceous feld- 
spathic rocks would thus measure between 800 and 900 feet. If a simple anticline be 
supposed it would measure about 1,300 feet, and if a monocline, as represented by 
E. Hitchcock in his Massachusetts section, about 2,600 feet.^ 

The rocks of Stone hill are frequently jointed; one of the systems of joints may 
possibly be connected with the pitch, as may also the occasional east to west joints 
and some of the secondary cleavage planes on Greylock. On the east side of the 
southern portion of the hill the massive quartzite is traversed by joints striking 
north 65° east, and dipping 05° to 75° northwesterly. On the east side of the central 
part the micaceous quartzite has a set of joints striking north 80^ east and dipi)ing 
80° southerly, and another set striking north L'Oo west, and dipping 45° easterly. The 
dark pyritiferous quartzite (locality 18) near the top and center has joints striking 
north 72° east, and dipjting 65° north-northwest. 



'See his Section 46 (Geology second fUstriet, New York, p. 145), in wUicli lie repi'csents a I'ault 
immediately eiist of Stone hill, and another farther east along the western foot of the Greylock range. 
^ Dewey refers to the contortions here: Am. Jour, of Sci., ser. i, vol. 9, 1825, p. 18. 
' Report Gaol, of Vermont, vol. 2, pi. xv, fig. 5. 



200 GREEN MOUNTAINS IN MASSACHUSETTS. 

The following is Mr. J. E. Wolflf's summary of his notes on the Stone hill micro- 
scopic sections: 

STONE HILL ROCKS. 

"We have in the quartzite series of Stone hill an interesting illustration of the 
share that the original detritus and the modification produced by mechanical and 
chemical agencies take in producing certain rocks. 

"The quartzite varies microscopically from a fine-grained rock, composed to the 
eye of quartz grains and more or less mica to a coarse fragmental quartzite or fine- 
grained conglomerate (locality 628) in which angular fragments of feldspar and rather 
rounded masses or pebbles of blue quartz are visible; the latter grade insensibly into 
the granular white quartz forming the rest of the rock. 

"Studied in the thin section the structure of the rocks is as follows: The large 
masses of blue quartz show in polarized light that they have been subjected to great 
pressiu'e and strain, which has resulted in a partial or total breaking up of the original 
homogeneous quartz into a 'groundmass' or mosaic composed of extremely small 
particles of quartz in which are contained cores of cracked and strained quartz which 
are remnants of the original masses.' The comparatively large fragments of feld- 
spar are seen to be in most cases microcline or a plagioclase feldspar, but sometimes 
without evidence of multiple twinning, and in that case probably orthoclase. The 
substance of the feldspar is cloudy, owing to kaolinization. The forms are sharj)ly 
angular and evidently detrital. The remainder of the rock is a very fine-grained aggre- 
gate of little grains of quartz and rarer ones of feldspar, the latter being similar in 
character to the larger fragments of the same mineral. Irregular and interrupted 
layers of a colorless muscovite, which has the wavy 'interwoven' structural form 
characteristic of sericite, give the rock a lamination, the plane of which is parallel 
to the planes of crashing in the quartz, that is, at right angles to the pressure. When 
one of these layers of mica touches one of the large clastic feldspars, it often forks and 
completely siu-rounds the feldspar, the two parts joining again on the other side; 
accompanying this there is a thickening of the layer of mica around and near the feld- 
spar, and sometimes litttle tongues of the mica, branching from the main mass outside, 
penetrate the feldspar, especially along cleavage cracks. It is therefore evident that 
the clastic feldspar exercised an influence on the formation of the mica and probably 
gave up part of its substance to form the latter. These large feldspars, like the 
quartz, are fractured and broken, the quartz aggregate of the 'groundmass' filling 
the fissures. 

"The small feldspars of the 'groundmass' have in part the same characters as 
the large detrital ones, and in fact are often evidently derived from an adjacent large 

' See PI. X in Part il for an enlarged photograph of a thin section of this cruehed blue quartz 
from Stone hill. > 



STONE HILL. 201 

grain, but in part they have a more rounded form and show little trace of decompo- 
sition. In some of these grains there is a central core which is opaque owing to 
kaolinization (as is the case with the whole grain in the case of the large fragments) 
but surrounded by an outer rim of clear fresh feldspar material, which has the same 
crystallographic orientation as the inner core, the two forming one grain. If these 
grains are detrital, as they seem to be, there must have been a recrystallization of the 
old feldspar or a deposition of new feldspar around the old grain. ^ 

"In certain finegrained varieties of these Stone hill quartzites the amount of 
feldspar is very large, and it is difficult to say whether these small grains are in their 
original detrital shape or are metamorphic. 

" In sojae cases the large clastic feldspar masses are aggregates of several individ- 
ual grains of feldspar, forming thus a rock fragment which resembles closely the 
coarse granitoid gneiss found on Clarksburg mountain to the northeast and Hoosac 
mountain to the east, which undei'lies the whole Taconic series. Hence there is a pos- 
sible derivation for the material of the quartzite. 

" Prisms of tourmaline are common in the rock, and there are occasional rounded 
grains of zircon. Secondary limonite often stains the rock yellow. Grains of pyrite 
are abundant in some specimens (locality IS, near top of hill), and in one there is a 
large amount of calcite present in small grains and irregular masses. 

"These quartzites seem to derive their present materials from two sources, the 
original detrital material and the material produced from this, at least in part, by 
mechanical and chemical agencies. The blue quartz 'pebbles' (locality 628, east side) 
may be regarded as pebbles whose original outlines have been largely obliterated by 
mechanical deformation ; the large feldspar fragments are undoubtedly detrital and 
so is the zircon. The cement or 'groundmass' is composed of detrital quartz and 
leldsijar mixed with an unknown amount of the same minei-als formed in situ and by 
muscovite in large part and tourmaline produced by metamorphism. 

"The distinction made here between clastic and metamorphic feldspar is well 
marked in the extremes as found on the clastic side in these rocks; on the metamor- 
phic side in the albite of the schists of Greylock and Hoosac mountains, and analo- 
gous feldspars of the gneisses of Hoosac mountain." 

'Cf. Irving and Van Hise, Bull. U. S. Geol. Survey No. 8, p. 44. 



APPENDIX B. 



NEW ASHFORD. 




Fig. 77.— Apex of the main anticline of Stockbridge 
limestone protruding ttimugli the Berkshire schist at 
Quarry hill, New A&hford. Height, 6 feet. Southern 
side. See locality 296, Fig. 78. 



There is an area of between 3 and 4 square miles south and east of the village of 
New Ashford, within which nearly all the structural and areal features that char- 
acterize the Greylock mass are repeated on a 
small scale and within easy reach. PI. i shows 
the geology of this tract. Section M traverses 
it. Fig. 74 gives a view of the greater portion 
of it and of Sugarloaf mountain which covers 
a large part 'of the area. This little schist 
mountain, the synclinal struc';urD of which 
has already been alluded to, is entirely sur- 
rounded as well as underlain by limestone. 
It forms a conspicuous object in the land- 
scape, views of it from the north (Fig. 30) and 
the south (PL xv) showing the depression on 
either side of it corresponding to the limestone. 
A line of cliffs, masked, however, by foliage, traverses its south end from east 
to west, rising above the limestone which pitches under it. On the west side of 
Sugarloaf the synclinal structure is concealed in most of 
the limestone out-cro]is by cleavage foliation. (See Fig. 37.) 
A northerly pitch is well observed at the south end in some 
of the minor folds (see Fig. 60), as well as a southerly pitch 
in the schist at the north end. Section II", which follows 
the synclinal axis of Sugarloaf, shows the trough structure 
of that mountain. Another trough exists in the schist mass 
south of it. 

Several isolated schist masses cap the limestone folds 
along the foot of the mountain on the south. The phenomena 
of cleavage and stratification in one of these have been shown in Fig. 35. 

On Quarry hill the converse of the structure presented by Sugarloaf mountain 
appears. A limestone anticline with subordinate folds protrudes through the schist. 
202 • 




Fig. 78 Geologic map of 

Quarry hill. New Ashford. 



NEW ASHFOUl). 



203 



The diaigraiiis (Figs. 77, 78, 7'.») leprcseut the area, size, and structm-e of this anticline, 
and Figs. 32, 33, 3i show the cleavage phenomena iu it. 

The schists at the foot of the hill toward the village form part of those of East 
mountain (Beach hill). The easterly cleavage would easily mislead oue here into a 
wrong interpretation of the relations. The broad area of limestone in which the old 



Ashford Schisl Sb 3S'S-£ SChtst +5" S £ SchiStSb 

''■'"" r' QUARRT Hiltl 



:7"''12;^4s.s/i^^^;^!'!^i 




Fig. 70. — Section through Quarry hill, Xt;\v Ashlbrrt, showing the structural relations of the Stockbridge limestone 
and Berkshire Hchist. 

quarries lie, forms an anticline, and the schists referred to overlie its base with a 
westerly dip. It is uncertain whether the section given by Emmons (Geology of sec- 
ond district, p. 155), through the New Ashford marble quarry, relates to this quarry 
or to one of several others iu the vicinity. 



INDEX 



Page. 

Adam9,Mas9., exposures of gneiss near 84 

Ampbibolites, areas and character of 65-66 

Anthonys creek, exposures on s4 

Ausweichungsclivage of Hoim 139 

Bald mountain, figured specimen of schist from. ... 144 

Baltzer, A., cited 143 

"Bellowspipe," location of 160 

Bellowspipe limestone, thickness of 20 

age and character of 180, 184-186 

Berkshire schist, exposures of 08 

syncline of, in Cheshire 15, 16 

thickness of 20 

age and character of 179, 182-184 

Berkshire valley described 6 

Bowens creek, section on 85 

Burlingames hill, exposures at 85 

" Buttress," location and structure of 22 

exposures of gneiss at 83 

Cadell, H. M., cited, 179 

Cambrian quartzite correlated with Hoosac conglom- 
erate 28.29 

Cambrian and pre-Cambrian rocks not easily distin- 
guished 25 

Cambrian rocks, varying character of 31 

Cheshire, schist and limestone in 16 

ideal section near 17 

Cheshire hills, transition from limestone to schist at 

south end of 16, 17 

Chester ampbibolites, geologic place of 30 

Clarksburg mountain, structure of 8-9, 10, 27, 176 

rocks of 26,27 

exposures at 99 

Cleavage, list of works on 137, 138 

nature of. Mount Greylock 158 

Cleavage and stratification foliations, relations of. . 136-137, 
139, 140, 141, 144-155, 155-157, 161, 173 

Connecticut valley described 6 

Conway schist, place of 30 

Cook, George H., cited 157 

Correlation of Green mountain rocks 9, 35 

Dale, T. Nelson, work of xiv, 12, 19-20 

paper on Mount Greylock by 119-203 

cited 191 

Dalton, exposures in 96 

Dalton-Windsor hills, structure of 16 

Dana, J. D., cited 9, 50, 1 07, 

131, 132, 155, 157, 159, 163, 169, 182, 188, 189, 190, 193 

Darwin, C, cited 143 

Deer hill, location of 135 

Dewey, Chester, cited 131 , 163, 181, 198 

Dry brook, exposures near 91, 93, 94, 96, 98 

East mountain, specimen of limestone from 142 

figured specimen of schist from 146 

Eaton, Amos, cited 131 

Emerson, B. K., work of 10, 26, 30 

Emmons, B., cited 14, 1U7, 131, 

132, 159, 163, 164, 181, 184, 185, 190, 197. 198, 199 



Page. 

Favre, A Iphonse, cited 179 

Feldspar, analysis of 187 

Formations, table of 190 

Geology and topography, relations of 192-196 

Greylock schist, thickness of 20 

age and character of 180,186 

Greylock and Hoosac rocks correlated 13-20 

Hall, James, cited 108,132,159,190 

Hawes, G. W., cited 67 

Heim, A., cited 106,139 

Hitchcock, C. H , cited 7,58,100,107 

Hitchcock, E., cited 67,107,131, 

132, 159, 164, 181, 182, 183, 184, 199 

Hobbs, W. H., work of xvi, 131 

aid by 12 

cited 180 

Hoosac conglomerate correlated with Cambrian 

quartzite 28, 29 

Hoosac mountain, structure of 8, 20, 21, 80, 104-106 

rocks of 26,44-69,102 

report of J. E. Wolfi' on 35-118 

general topography of 41-44 

Hoosac tunnel, geologic value of -■ 8 

described 8.9,42,69-72 

Hoosac schist, exposures of, in tunnel 23,69,72 

described 59 

exposures of 72,80,81,82,87,88 

Hoosac schist and Stockbridge limestone, zone of 

lateral transition between 15, 17 

Hoosac and Greylock rocks correlated 13-20 

Hoosic river, exposures on 87 

Hoosic valley schist 97-98 

*' Hopper," location of 134 

Hunt, T. Sterry, cited 108 

Irving and Van Hise, cited 201 

Jukes, J. B., cited 143 

Lachines creek, contact of Stockbridge limestone 

and Cambrian quartzite on 12 

Logan, "W. E., cited '. 7 

Marcou, Jules, cited 191 

Metamorphism, phases of 32, 33, 34 

Mount Greylock, structure of 21, 125-127, 177-1 79 

structure of rocks of 102, 127-128, 136-155, 181 

paper by T. Nelson Dale on 119-203 

description of 125,133-136 

areal geology of 128 

relations of geology to topography on 128, 129 

figured specimens of schist from 145,147 

pitch of folds of 175 

table of formation s of 190 

Mount Prospect (Symonds peak), location of 134,135 

figured specimens of schist from 144, 145 

structure of 162-163 

New Ashford, geology of area near 202-203 

North Adams, exposures at 87-88, 98 

205 



20*) 



INDEX. 



"Notch," location of 

( ileuelhis casts tniiml 

Pierce, Josiali, ;iid by 

Pitili, methoils of dtterioininji, 

general. Mount (Ireylock 

riaiiilieUl schist, geoloj;ic phice *if- 



Page. 

100 

10, 29 

siv 

157 
175 

:jo 



PniigiiqiiaK, N. Y., contact of old gneiss and Cam- 
brian quartzite at 21i 

PunipeUy, K., cited 182 

Putnam. B. T., work of xiii-xiv. 8, 10,21 

(Quarry bill. New Asbford, exposures at 138, 139, UO 

Ragged mountain, location of 135 

topography and structure of iSO, 170, 171.175,170 

Renard, A., cited 183 

Reusch, H., cited 143, 102 

Richthofen, F. von, cited 27 

Riggs, R. B., analysis of felds]>ar by GO, 186-187 

Rosenbusch, H., cited 63 

Rowe schist, doubtful age of ■ 29 

place of 30 

area of 65 

Saddle Ball, location and height of 135 

Savoy Center, outcrop of white gneiss conglomerate 

near 79 

Savoy Hollow, outcrops near 78, 79 

Southwick creek, stratigraphy on 77 

Spruce hill, exposures near 86-87,88 

Stamford, Vt., expi>sures near 98-102 

contact of Stamford gneiss and Vermont quartz- 
ite at 100, 101 

dike in 11 

Stamford gneiss, description of 45-48 

fossils of 51 

exposures of, in Eoosac tunnel 69,72 

contact of, with conglomerate of Vermont for- 
mation 73, 100 

Stockbridge limestone, syncline of, in Cheshire 15 

thickness of 20 

description of 64-65 



Page. 
Stockbridge limestone— Continued. 

exposures of, in Hoosac tunnel 69.72 

exposures of 84, 87, 89, 98 

age and character of 179, 181-182 

Stockbridge limestone and Hoosac schist, zone of 

''lateral transition between 15, 17 

Stone bill, geology and topography of 197-201 

Strata of Mount Greylock. table 190 

Stratiiicalion and cleavage foliations, relations of. 

at Mount Greylock 136,137 

Stratification foliation, character of 158 

Sugar loaf mountain, strata at 140, 141 

structure of 173 

Symonds peak (Mount Prospect), structure of 162-163 

Tacnnic mountains described 6 

Topbet creek, exposures of Hoosac schist on. 81-82 

exposures of rocks of Vermont formaticui on 84. H5 

Topographic work on Hoosac mountain, methods 

of 41 

Topography and geology, relations of 192-196 

Vermont formation described 48-59 

exposures of, in Hoosac^ tunnel 69, 72 

exposures of 72, 82-88, 88-90, 94 

contact of, with Stamfoi'fl gneiss 73 

age and character of , 179, 200-201 

Vermont quartzite and Stockbridge limestone, fea- 
tures of contact of 95-96 

Walcott, C. D., aid by xiv. 10, 29 

fossils found in Stamford gneiss by 51 

cited 163. 190, 191 

Whittle, C. L., aid by xiv 

Winchell. A .. cited 58 

Windsor, Vt .exposures at 96 

Windsor hill, exposures at 88 

Wolfl; J. E , wtuk of xiii. 8, 10, 11, 14, 28, 125 

cited 164, 171. 183, 187, 191 

quoted on microscopic sections of Stone bill 

rocks 200 

Vokura, Mr., aid by xiv